diff --git a/pkg/app/piped/eventwatcher/BUILD.bazel b/pkg/app/piped/eventwatcher/BUILD.bazel
new file mode 100644
index 0000000000..2e74e37523
--- /dev/null
+++ b/pkg/app/piped/eventwatcher/BUILD.bazel
@@ -0,0 +1,15 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["eventwatcher.go"],
+    importpath = "github.com/pipe-cd/pipe/pkg/app/piped/eventwatcher",
+    visibility = ["//visibility:public"],
+    deps = [
+        "//pkg/config:go_default_library",
+        "//pkg/git:go_default_library",
+        "//pkg/model:go_default_library",
+        "//pkg/yamlprocessor:go_default_library",
+        "@org_uber_go_zap//:go_default_library",
+    ],
+)
diff --git a/pkg/app/piped/eventwatcher/eventwatcher.go b/pkg/app/piped/eventwatcher/eventwatcher.go
new file mode 100644
index 0000000000..9e3fef8868
--- /dev/null
+++ b/pkg/app/piped/eventwatcher/eventwatcher.go
@@ -0,0 +1,254 @@
+// Copyright 2021 The PipeCD Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package eventwatcher provides facilities to update config files when new
+// event found. It can be done by periodically comparing the latest value user
+// registered and the value in the files placed at Git repositories.
+package eventwatcher
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"sync"
+	"time"
+
+	"go.uber.org/zap"
+
+	"github.com/pipe-cd/pipe/pkg/config"
+	"github.com/pipe-cd/pipe/pkg/git"
+	"github.com/pipe-cd/pipe/pkg/model"
+	"github.com/pipe-cd/pipe/pkg/yamlprocessor"
+)
+
+const (
+	// The latest value and Event name are supposed.
+	defaultCommitMessageFormat = "Replace values with %q set by Event %q"
+	defaultCheckInterval       = 5 * time.Minute
+)
+
+type Watcher interface {
+	Run(context.Context) error
+}
+
+type eventGetter interface {
+	GetLatest(ctx context.Context, name string, labels map[string]string) (*model.Event, bool)
+}
+
+type gitClient interface {
+	Clone(ctx context.Context, repoID, remote, branch, destination string) (git.Repo, error)
+}
+
+type commit struct {
+	changes map[string][]byte
+	message string
+}
+
+type watcher struct {
+	config      *config.PipedSpec
+	eventGetter eventGetter
+	gitClient   gitClient
+	logger      *zap.Logger
+	wg          sync.WaitGroup
+}
+
+func NewWatcher(cfg *config.PipedSpec, eventGetter eventGetter, gitClient gitClient, logger *zap.Logger) Watcher {
+	return &watcher{
+		config:      cfg,
+		eventGetter: eventGetter,
+		gitClient:   gitClient,
+		logger:      logger.Named("event-watcher"),
+	}
+}
+
+// Run spawns goroutines for each git repository. They periodically fetch the latest Event
+// from the control-plane to compare the value with one in the git repository.
+func (w *watcher) Run(ctx context.Context) error {
+	w.logger.Info("start running event watcher")
+
+	for _, repoCfg := range w.config.Repositories {
+		repo, err := w.gitClient.Clone(ctx, repoCfg.RepoID, repoCfg.Remote, repoCfg.Branch, "")
+		if err != nil {
+			w.logger.Error("failed to clone repository",
+				zap.String("repo-id", repoCfg.RepoID),
+				zap.Error(err),
+			)
+			return fmt.Errorf("failed to clone repository %s: %w", repoCfg.RepoID, err)
+		}
+		defer os.RemoveAll(repo.GetPath())
+
+		w.wg.Add(1)
+		go w.run(ctx, repo, &repoCfg)
+	}
+
+	w.wg.Wait()
+	return nil
+}
+
+// run works against a single git repo. It periodically compares the value in the given
+// git repository and one in the control-plane. And then pushes those with differences.
+func (w *watcher) run(ctx context.Context, repo git.Repo, repoCfg *config.PipedRepository) {
+	defer w.wg.Done()
+
+	var (
+		commitMsg                  string
+		includedCfgs, excludedCfgs []string
+	)
+	// Use user-defined settings if there is.
+	for _, r := range w.config.EventWatcher.GitRepos {
+		if r.RepoID != repoCfg.RepoID {
+			continue
+		}
+		commitMsg = r.CommitMessage
+		includedCfgs = r.Includes
+		excludedCfgs = r.Excludes
+		break
+	}
+	checkInterval := time.Duration(w.config.EventWatcher.CheckInterval)
+	if checkInterval == 0 {
+		checkInterval = defaultCheckInterval
+	}
+
+	ticker := time.NewTicker(checkInterval)
+	defer ticker.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-ticker.C:
+			err := repo.Pull(ctx, repo.GetClonedBranch())
+			if err != nil {
+				w.logger.Error("failed to perform git pull",
+					zap.String("repo-id", repoCfg.RepoID),
+					zap.String("branch", repo.GetClonedBranch()),
+					zap.Error(err),
+				)
+				continue
+			}
+			cfg, err := config.LoadEventWatcher(repo.GetPath(), includedCfgs, excludedCfgs)
+			if errors.Is(err, config.ErrNotFound) {
+				w.logger.Info("configuration file for Event Watcher not found",
+					zap.String("repo-id", repoCfg.RepoID),
+					zap.Error(err),
+				)
+				continue
+			}
+			if err != nil {
+				w.logger.Error("failed to load configuration file for Event Watcher",
+					zap.String("repo-id", repoCfg.RepoID),
+					zap.Error(err),
+				)
+				continue
+			}
+			if err := w.updateValues(ctx, repo, cfg.Events, commitMsg); err != nil {
+				w.logger.Error("failed to update the values",
+					zap.String("repo-id", repoCfg.RepoID),
+					zap.Error(err),
+				)
+			}
+		}
+	}
+}
+
+// updateValues inspects all Event-definition and pushes the changes to git repo if there is.
+func (w *watcher) updateValues(ctx context.Context, repo git.Repo, events []config.EventWatcherEvent, commitMsg string) error {
+	// Copy the repo to another directory to avoid pull failure in the future.
+	tmpDir, err := ioutil.TempDir("", "event-watcher")
+	if err != nil {
+		return fmt.Errorf("failed to create a new temporary directory: %w", err)
+	}
+	defer os.RemoveAll(tmpDir)
+	tmpRepo, err := repo.Copy(filepath.Join(tmpDir, "tmp-repo"))
+	if err != nil {
+		return fmt.Errorf("failed to copy the repository to the temporary directory: %w", err)
+	}
+
+	commits := make([]*commit, 0)
+	for _, e := range events {
+		c, err := w.modifyFiles(ctx, &e, tmpRepo, commitMsg)
+		if err != nil {
+			w.logger.Error("failed to check outdated value", zap.Error(err))
+			continue
+		}
+		if c != nil {
+			commits = append(commits, c)
+		}
+	}
+	if len(commits) == 0 {
+		return nil
+	}
+
+	w.logger.Info(fmt.Sprintf("there are %d outdated values", len(commits)))
+	for _, c := range commits {
+		if err := tmpRepo.CommitChanges(ctx, tmpRepo.GetClonedBranch(), c.message, false, c.changes); err != nil {
+			return fmt.Errorf("failed to perform git commit: %w", err)
+		}
+	}
+	return tmpRepo.Push(ctx, tmpRepo.GetClonedBranch())
+}
+
+// modifyFiles modifies files defined in a given Event if any deviation exists between the value in
+// the git repository and one in the control-plane. And gives back a change contents.
+func (w *watcher) modifyFiles(ctx context.Context, event *config.EventWatcherEvent, repo git.Repo, commitMsg string) (*commit, error) {
+	latestEvent, ok := w.eventGetter.GetLatest(ctx, event.Name, event.Labels)
+	if !ok {
+		return nil, fmt.Errorf("failed to get the latest Event with the name %q", event.Name)
+	}
+
+	// Determine files to be changed.
+	changes := make(map[string][]byte, 0)
+	for _, r := range event.Replacements {
+		path := filepath.Join(repo.GetPath(), r.File)
+		yml, err := ioutil.ReadFile(path)
+		if err != nil {
+			return nil, fmt.Errorf("failed to read file: %w", err)
+		}
+		v, err := yamlprocessor.GetValue(yml, r.YAMLField)
+		if err != nil {
+			return nil, fmt.Errorf("failed to get value at %s in %s: %w", r.YAMLField, r.File, err)
+		}
+		value, ok := v.(string)
+		if !ok {
+			return nil, fmt.Errorf("unknown type of value is defined at %s in %s", r.YAMLField, r.File)
+		}
+		if latestEvent.Data == value {
+			// Already up-to-date.
+			continue
+		}
+		// Modify the local file and put it into the change list.
+		newYml, err := yamlprocessor.ReplaceValue(yml, r.YAMLField, latestEvent.Data)
+		if err != nil {
+			return nil, fmt.Errorf("failed to replace value at %s with %s: %w", r.YAMLField, latestEvent.Data, err)
+		}
+		if err := ioutil.WriteFile(path, newYml, os.ModePerm); err != nil {
+			return nil, fmt.Errorf("failed to write file: %w", err)
+		}
+		changes[r.File] = newYml
+	}
+
+	if len(changes) == 0 {
+		return nil, nil
+	}
+
+	if commitMsg == "" {
+		commitMsg = fmt.Sprintf(defaultCommitMessageFormat, latestEvent.Data, event.Name)
+	}
+	return &commit{
+		changes: changes,
+		message: commitMsg,
+	}, nil
+}