diff --git a/deployment/command-line-option.md b/deployment/command-line-option.md
index f88e471b..473b2680 100644
--- a/deployment/command-line-option.md
+++ b/deployment/command-line-option.md
@@ -21,6 +21,7 @@ Usage: fluentd [options]
         --workers NUM                specify the number of workers under supervisor
         --user USER                  change user
         --group GROUP                change group
+        --umask UMASK                change umask
     -o, --log PATH                   log file path
         --log-rotate-age AGE         generations to keep rotated log files
         --log-rotate-size BYTES      sets the byte size to rotate log files
diff --git a/deployment/system-config.md b/deployment/system-config.md
index 7322c9a3..0aea62aa 100644
--- a/deployment/system-config.md
+++ b/deployment/system-config.md
@@ -238,6 +238,7 @@ For code example, please refer to [`api-plugin-base`](../plugin-development/api-
 * `--workers NUM`: Specifies the number of workers under supervisor.
 * `--user USER`: Changes user.
 * `--group GROUP`: Changes group.
+* `--umask UMASK`: Specifies umask to use, in octal form. Defaults to 0.
 * `-o`, `--log PATH`: Specifies the log file path.
 * `--log-rotate-age AGE`: Specifies the generations to keep rotated log files.
 * `--log-rotate-size BYTES`: Sets the byte size to rotate log files.