diff --git a/pkg/app/piped/cloudprovider/terraform/module.go b/pkg/app/piped/cloudprovider/terraform/module.go
index 205e06cd5f..fed3d86ddc 100644
--- a/pkg/app/piped/cloudprovider/terraform/module.go
+++ b/pkg/app/piped/cloudprovider/terraform/module.go
@@ -22,7 +22,7 @@ type FileMapping struct {
 type ModuleMapping struct {
 	Name    string   `hcl:"name,label"`
 	Source  string   `hcl:"source"`
-	Version string   `hcl:"version"`
+	Version string   `hcl:"version,optional"`
 	Remain  hcl.Body `hcl:",remain"`
 }
 
diff --git a/pkg/app/piped/cloudprovider/terraform/module_test.go b/pkg/app/piped/cloudprovider/terraform/module_test.go
index 5e8453bb93..6fc0d3cae4 100644
--- a/pkg/app/piped/cloudprovider/terraform/module_test.go
+++ b/pkg/app/piped/cloudprovider/terraform/module_test.go
@@ -34,6 +34,22 @@ func TestLoadTerraformFiles(t *testing.T) {
 			},
 			expectedErr: false,
 		},
+		{
+			name:      "single module with optional argument",
+			moduleDir: "./testdata/single_module_optional",
+			expected: []File{
+				{
+					Modules: []*Module{
+						{
+							Name:    "helloworld",
+							Source:  "helloworld",
+							Version: "",
+						},
+					},
+				},
+			},
+			expectedErr: false,
+		},
 		{
 			name:      "multi modules",
 			moduleDir: "./testdata/multi_modules",
@@ -120,6 +136,19 @@ func TestFindArticatVersions(t *testing.T) {
 			},
 			expectedErr: false,
 		},
+		{
+			name:      "single module with optional field",
+			moduleDir: "./testdata/single_module_optional",
+			expected: []*model.ArtifactVersion{
+				{
+					Kind:    model.ArtifactVersion_TERRAFORM_MODULE,
+					Name:    "helloworld",
+					Url:     "helloworld",
+					Version: "",
+				},
+			},
+			expectedErr: false,
+		},
 		{
 			name:      "multi modules",
 			moduleDir: "./testdata/multi_modules",
diff --git a/pkg/app/piped/cloudprovider/terraform/testdata/single_module_optional/helloworld/main.tf b/pkg/app/piped/cloudprovider/terraform/testdata/single_module_optional/helloworld/main.tf
new file mode 100644
index 0000000000..6dadf55b5b
--- /dev/null
+++ b/pkg/app/piped/cloudprovider/terraform/testdata/single_module_optional/helloworld/main.tf
@@ -0,0 +1,19 @@
+resource "docker_container" "helloworld" {
+  name = "gcr.io/pipecd/helloworld:${var.image_version}"
+  ports {
+    internal = 9376
+    external = "${var.external_port}"
+  }
+}
+
+variable "external_port" {
+    default = 80
+}
+
+variable "image_version" {
+  default = "latest"
+}
+
+output "container" {
+  value = docker_image.helloworld
+}
diff --git a/pkg/app/piped/cloudprovider/terraform/testdata/single_module_optional/main.tf b/pkg/app/piped/cloudprovider/terraform/testdata/single_module_optional/main.tf
new file mode 100644
index 0000000000..2ab0a5fa49
--- /dev/null
+++ b/pkg/app/piped/cloudprovider/terraform/testdata/single_module_optional/main.tf
@@ -0,0 +1,8 @@
+provider "docker" {
+}
+
+module "helloworld" {
+  source = "helloworld"
+  image_version = "v1.0.0"
+  external_port = 8080
+}
diff --git a/pkg/model/common.pb.go b/pkg/model/common.pb.go
index 4918c9a4ea..8438d03941 100644
--- a/pkg/model/common.pb.go
+++ b/pkg/model/common.pb.go
@@ -643,39 +643,38 @@ var file_pkg_model_common_proto_rawDesc = []byte{
 	0x1a, 0x39, 0x0a, 0x0b, 0x4c, 0x61, 0x62, 0x65, 0x6c, 0x73, 0x45, 0x6e, 0x74, 0x72, 0x79, 0x12,
 	0x10, 0x0a, 0x03, 0x6b, 0x65, 0x79, 0x18, 0x01, 0x20, 0x01, 0x28, 0x09, 0x52, 0x03, 0x6b, 0x65,
 	0x79, 0x12, 0x14, 0x0a, 0x05, 0x76, 0x61, 0x6c, 0x75, 0x65, 0x18, 0x02, 0x20, 0x01, 0x28, 0x09,
-	0x52, 0x05, 0x76, 0x61, 0x6c, 0x75, 0x65, 0x3a, 0x02, 0x38, 0x01, 0x22, 0xf4, 0x01, 0x0a, 0x0f,
+	0x52, 0x05, 0x76, 0x61, 0x6c, 0x75, 0x65, 0x3a, 0x02, 0x38, 0x01, 0x22, 0xeb, 0x01, 0x0a, 0x0f,
 	0x41, 0x72, 0x74, 0x69, 0x66, 0x61, 0x63, 0x74, 0x56, 0x65, 0x72, 0x73, 0x69, 0x6f, 0x6e, 0x12,
 	0x39, 0x0a, 0x04, 0x6b, 0x69, 0x6e, 0x64, 0x18, 0x01, 0x20, 0x01, 0x28, 0x0e, 0x32, 0x1b, 0x2e,
 	0x6d, 0x6f, 0x64, 0x65, 0x6c, 0x2e, 0x41, 0x72, 0x74, 0x69, 0x66, 0x61, 0x63, 0x74, 0x56, 0x65,
 	0x72, 0x73, 0x69, 0x6f, 0x6e, 0x2e, 0x4b, 0x69, 0x6e, 0x64, 0x42, 0x08, 0xfa, 0x42, 0x05, 0x82,
-	0x01, 0x02, 0x10, 0x01, 0x52, 0x04, 0x6b, 0x69, 0x6e, 0x64, 0x12, 0x21, 0x0a, 0x07, 0x76, 0x65,
-	0x72, 0x73, 0x69, 0x6f, 0x6e, 0x18, 0x02, 0x20, 0x01, 0x28, 0x09, 0x42, 0x07, 0xfa, 0x42, 0x04,
-	0x72, 0x02, 0x10, 0x01, 0x52, 0x07, 0x76, 0x65, 0x72, 0x73, 0x69, 0x6f, 0x6e, 0x12, 0x12, 0x0a,
-	0x04, 0x6e, 0x61, 0x6d, 0x65, 0x18, 0x03, 0x20, 0x01, 0x28, 0x09, 0x52, 0x04, 0x6e, 0x61, 0x6d,
-	0x65, 0x12, 0x10, 0x0a, 0x03, 0x75, 0x72, 0x6c, 0x18, 0x04, 0x20, 0x01, 0x28, 0x09, 0x52, 0x03,
-	0x75, 0x72, 0x6c, 0x22, 0x5d, 0x0a, 0x04, 0x4b, 0x69, 0x6e, 0x64, 0x12, 0x0b, 0x0a, 0x07, 0x55,
-	0x4e, 0x4b, 0x4e, 0x4f, 0x57, 0x4e, 0x10, 0x00, 0x12, 0x13, 0x0a, 0x0f, 0x43, 0x4f, 0x4e, 0x54,
-	0x41, 0x49, 0x4e, 0x45, 0x52, 0x5f, 0x49, 0x4d, 0x41, 0x47, 0x45, 0x10, 0x01, 0x12, 0x0d, 0x0a,
-	0x09, 0x53, 0x33, 0x5f, 0x4f, 0x42, 0x4a, 0x45, 0x43, 0x54, 0x10, 0x02, 0x12, 0x0e, 0x0a, 0x0a,
-	0x47, 0x49, 0x54, 0x5f, 0x53, 0x4f, 0x55, 0x52, 0x43, 0x45, 0x10, 0x03, 0x12, 0x14, 0x0a, 0x10,
-	0x54, 0x45, 0x52, 0x52, 0x41, 0x46, 0x4f, 0x52, 0x4d, 0x5f, 0x4d, 0x4f, 0x44, 0x55, 0x4c, 0x45,
-	0x10, 0x04, 0x2a, 0x53, 0x0a, 0x0f, 0x41, 0x70, 0x70, 0x6c, 0x69, 0x63, 0x61, 0x74, 0x69, 0x6f,
-	0x6e, 0x4b, 0x69, 0x6e, 0x64, 0x12, 0x0e, 0x0a, 0x0a, 0x4b, 0x55, 0x42, 0x45, 0x52, 0x4e, 0x45,
-	0x54, 0x45, 0x53, 0x10, 0x00, 0x12, 0x0d, 0x0a, 0x09, 0x54, 0x45, 0x52, 0x52, 0x41, 0x46, 0x4f,
-	0x52, 0x4d, 0x10, 0x01, 0x12, 0x0a, 0x0a, 0x06, 0x4c, 0x41, 0x4d, 0x42, 0x44, 0x41, 0x10, 0x03,
-	0x12, 0x0c, 0x0a, 0x08, 0x43, 0x4c, 0x4f, 0x55, 0x44, 0x52, 0x55, 0x4e, 0x10, 0x04, 0x12, 0x07,
-	0x0a, 0x03, 0x45, 0x43, 0x53, 0x10, 0x05, 0x2a, 0x41, 0x0a, 0x17, 0x41, 0x70, 0x70, 0x6c, 0x69,
-	0x63, 0x61, 0x74, 0x69, 0x6f, 0x6e, 0x41, 0x63, 0x74, 0x69, 0x76, 0x65, 0x53, 0x74, 0x61, 0x74,
-	0x75, 0x73, 0x12, 0x0b, 0x0a, 0x07, 0x45, 0x4e, 0x41, 0x42, 0x4c, 0x45, 0x44, 0x10, 0x00, 0x12,
-	0x0c, 0x0a, 0x08, 0x44, 0x49, 0x53, 0x41, 0x42, 0x4c, 0x45, 0x44, 0x10, 0x01, 0x12, 0x0b, 0x0a,
-	0x07, 0x44, 0x45, 0x4c, 0x45, 0x54, 0x45, 0x44, 0x10, 0x02, 0x2a, 0x36, 0x0a, 0x0c, 0x53, 0x79,
-	0x6e, 0x63, 0x53, 0x74, 0x72, 0x61, 0x74, 0x65, 0x67, 0x79, 0x12, 0x08, 0x0a, 0x04, 0x41, 0x55,
-	0x54, 0x4f, 0x10, 0x00, 0x12, 0x0e, 0x0a, 0x0a, 0x51, 0x55, 0x49, 0x43, 0x4b, 0x5f, 0x53, 0x59,
-	0x4e, 0x43, 0x10, 0x01, 0x12, 0x0c, 0x0a, 0x08, 0x50, 0x49, 0x50, 0x45, 0x4c, 0x49, 0x4e, 0x45,
-	0x10, 0x02, 0x42, 0x25, 0x5a, 0x23, 0x67, 0x69, 0x74, 0x68, 0x75, 0x62, 0x2e, 0x63, 0x6f, 0x6d,
-	0x2f, 0x70, 0x69, 0x70, 0x65, 0x2d, 0x63, 0x64, 0x2f, 0x70, 0x69, 0x70, 0x65, 0x63, 0x64, 0x2f,
-	0x70, 0x6b, 0x67, 0x2f, 0x6d, 0x6f, 0x64, 0x65, 0x6c, 0x62, 0x06, 0x70, 0x72, 0x6f, 0x74, 0x6f,
-	0x33,
+	0x01, 0x02, 0x10, 0x01, 0x52, 0x04, 0x6b, 0x69, 0x6e, 0x64, 0x12, 0x18, 0x0a, 0x07, 0x76, 0x65,
+	0x72, 0x73, 0x69, 0x6f, 0x6e, 0x18, 0x02, 0x20, 0x01, 0x28, 0x09, 0x52, 0x07, 0x76, 0x65, 0x72,
+	0x73, 0x69, 0x6f, 0x6e, 0x12, 0x12, 0x0a, 0x04, 0x6e, 0x61, 0x6d, 0x65, 0x18, 0x03, 0x20, 0x01,
+	0x28, 0x09, 0x52, 0x04, 0x6e, 0x61, 0x6d, 0x65, 0x12, 0x10, 0x0a, 0x03, 0x75, 0x72, 0x6c, 0x18,
+	0x04, 0x20, 0x01, 0x28, 0x09, 0x52, 0x03, 0x75, 0x72, 0x6c, 0x22, 0x5d, 0x0a, 0x04, 0x4b, 0x69,
+	0x6e, 0x64, 0x12, 0x0b, 0x0a, 0x07, 0x55, 0x4e, 0x4b, 0x4e, 0x4f, 0x57, 0x4e, 0x10, 0x00, 0x12,
+	0x13, 0x0a, 0x0f, 0x43, 0x4f, 0x4e, 0x54, 0x41, 0x49, 0x4e, 0x45, 0x52, 0x5f, 0x49, 0x4d, 0x41,
+	0x47, 0x45, 0x10, 0x01, 0x12, 0x0d, 0x0a, 0x09, 0x53, 0x33, 0x5f, 0x4f, 0x42, 0x4a, 0x45, 0x43,
+	0x54, 0x10, 0x02, 0x12, 0x0e, 0x0a, 0x0a, 0x47, 0x49, 0x54, 0x5f, 0x53, 0x4f, 0x55, 0x52, 0x43,
+	0x45, 0x10, 0x03, 0x12, 0x14, 0x0a, 0x10, 0x54, 0x45, 0x52, 0x52, 0x41, 0x46, 0x4f, 0x52, 0x4d,
+	0x5f, 0x4d, 0x4f, 0x44, 0x55, 0x4c, 0x45, 0x10, 0x04, 0x2a, 0x53, 0x0a, 0x0f, 0x41, 0x70, 0x70,
+	0x6c, 0x69, 0x63, 0x61, 0x74, 0x69, 0x6f, 0x6e, 0x4b, 0x69, 0x6e, 0x64, 0x12, 0x0e, 0x0a, 0x0a,
+	0x4b, 0x55, 0x42, 0x45, 0x52, 0x4e, 0x45, 0x54, 0x45, 0x53, 0x10, 0x00, 0x12, 0x0d, 0x0a, 0x09,
+	0x54, 0x45, 0x52, 0x52, 0x41, 0x46, 0x4f, 0x52, 0x4d, 0x10, 0x01, 0x12, 0x0a, 0x0a, 0x06, 0x4c,
+	0x41, 0x4d, 0x42, 0x44, 0x41, 0x10, 0x03, 0x12, 0x0c, 0x0a, 0x08, 0x43, 0x4c, 0x4f, 0x55, 0x44,
+	0x52, 0x55, 0x4e, 0x10, 0x04, 0x12, 0x07, 0x0a, 0x03, 0x45, 0x43, 0x53, 0x10, 0x05, 0x2a, 0x41,
+	0x0a, 0x17, 0x41, 0x70, 0x70, 0x6c, 0x69, 0x63, 0x61, 0x74, 0x69, 0x6f, 0x6e, 0x41, 0x63, 0x74,
+	0x69, 0x76, 0x65, 0x53, 0x74, 0x61, 0x74, 0x75, 0x73, 0x12, 0x0b, 0x0a, 0x07, 0x45, 0x4e, 0x41,
+	0x42, 0x4c, 0x45, 0x44, 0x10, 0x00, 0x12, 0x0c, 0x0a, 0x08, 0x44, 0x49, 0x53, 0x41, 0x42, 0x4c,
+	0x45, 0x44, 0x10, 0x01, 0x12, 0x0b, 0x0a, 0x07, 0x44, 0x45, 0x4c, 0x45, 0x54, 0x45, 0x44, 0x10,
+	0x02, 0x2a, 0x36, 0x0a, 0x0c, 0x53, 0x79, 0x6e, 0x63, 0x53, 0x74, 0x72, 0x61, 0x74, 0x65, 0x67,
+	0x79, 0x12, 0x08, 0x0a, 0x04, 0x41, 0x55, 0x54, 0x4f, 0x10, 0x00, 0x12, 0x0e, 0x0a, 0x0a, 0x51,
+	0x55, 0x49, 0x43, 0x4b, 0x5f, 0x53, 0x59, 0x4e, 0x43, 0x10, 0x01, 0x12, 0x0c, 0x0a, 0x08, 0x50,
+	0x49, 0x50, 0x45, 0x4c, 0x49, 0x4e, 0x45, 0x10, 0x02, 0x42, 0x25, 0x5a, 0x23, 0x67, 0x69, 0x74,
+	0x68, 0x75, 0x62, 0x2e, 0x63, 0x6f, 0x6d, 0x2f, 0x70, 0x69, 0x70, 0x65, 0x2d, 0x63, 0x64, 0x2f,
+	0x70, 0x69, 0x70, 0x65, 0x63, 0x64, 0x2f, 0x70, 0x6b, 0x67, 0x2f, 0x6d, 0x6f, 0x64, 0x65, 0x6c,
+	0x62, 0x06, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x33,
 }
 
 var (
diff --git a/pkg/model/common.pb.validate.go b/pkg/model/common.pb.validate.go
index 0d70e418c2..0a48f6ceb5 100644
--- a/pkg/model/common.pb.validate.go
+++ b/pkg/model/common.pb.validate.go
@@ -513,16 +513,7 @@ func (m *ArtifactVersion) validate(all bool) error {
 		errors = append(errors, err)
 	}
 
-	if utf8.RuneCountInString(m.GetVersion()) < 1 {
-		err := ArtifactVersionValidationError{
-			field:  "Version",
-			reason: "value length must be at least 1 runes",
-		}
-		if !all {
-			return err
-		}
-		errors = append(errors, err)
-	}
+	// no validation rules for Version
 
 	// no validation rules for Name
 
diff --git a/pkg/model/common.proto b/pkg/model/common.proto
index a1712adde7..6f1f88ca33 100644
--- a/pkg/model/common.proto
+++ b/pkg/model/common.proto
@@ -85,7 +85,7 @@ message ArtifactVersion {
     }
    
     Kind kind = 1 [(validate.rules).enum.defined_only = true];
-    string version = 2 [(validate.rules).string.min_len = 1];
+    string version = 2;
     string name = 3;
     string url = 4;
 }