diff --git a/ci.go b/ci.go
index e7a6f9e..2719296 100644
--- a/ci.go
+++ b/ci.go
@@ -155,7 +155,7 @@ func cloudbuild() (ci CI, err error) {
 	ci.PR.Number = 0
 	ci.PR.Revision = os.Getenv("COMMIT_SHA")
 
-	region := os.Getenv("REGION")
+	region := os.Getenv("_REGION")
 	if region == "" {
 		region = defaultCloudBuildRegion
 	}
diff --git a/ci_test.go b/ci_test.go
index 9313e11..1bf7e34 100644
--- a/ci_test.go
+++ b/ci_test.go
@@ -791,7 +791,7 @@ func TestCloudBuild(t *testing.T) {
 				os.Setenv("BUILD_ID", "build-id")
 				os.Setenv("PROJECT_ID", "gcp-project-id")
 				os.Setenv("_PR_NUMBER", "123")
-				os.Setenv("REGION", "asia-northeast1")
+				os.Setenv("_REGION", "asia-northeast1")
 			},
 			ci: CI{
 				PR: PullRequest{
@@ -808,7 +808,7 @@ func TestCloudBuild(t *testing.T) {
 				os.Setenv("BUILD_ID", "build-id")
 				os.Setenv("PROJECT_ID", "gcp-project-id")
 				os.Setenv("_PR_NUMBER", "")
-				os.Setenv("REGION", "")
+				os.Setenv("_REGION", "")
 			},
 			ci: CI{
 				PR: PullRequest{