diff --git a/operator_test.go b/operator_test.go
index 8106f666..00ca6225 100644
--- a/operator_test.go
+++ b/operator_test.go
@@ -408,8 +408,8 @@ func TestSkipIncluded(t *testing.T) {
 		skipIncluded bool
 		want         int
 	}{
-		{"testdata/book/include_*", false, 3},
-		{"testdata/book/include_*", true, 1},
+		{"testdata/book/include_*", false, 5},
+		{"testdata/book/include_*", true, 2},
 	}
 	for _, tt := range tests {
 		ops, err := Load(tt.paths, SkipIncluded(tt.skipIncluded), Runner("req", "https://api.github.com"), Runner("db", "sqlite://path/to/test.db"))
@@ -501,6 +501,7 @@ func TestInclude(t *testing.T) {
 		book string
 	}{
 		{"testdata/book/include_main.yml"},
+		{"testdata/book/include_vars.yml"},
 	}
 	ctx := context.Background()
 	for _, tt := range tests {
diff --git a/testdata/book/include_vars_included.yml b/testdata/book/include_vars_included.yml
new file mode 100644
index 00000000..ac391a50
--- /dev/null
+++ b/testdata/book/include_vars_included.yml
@@ -0,0 +1,21 @@
+desc: Include vars (included)
+if: included
+steps:
+  -
+    test: |
+      // If it is a primitive type, the value is passed as it is.
+      vars.a == 12345
+      && vars.b == "54321"
+      // If there is a variable expansion, it is reinterpreted in a form without quotes.
+      && vars.c == 12345
+      && vars.d == 54321
+      && vars.e == 123459
+      && vars.f == 543219
+      && vars.g == 912345
+      && vars.h == 954321
+      // If cast with string(), it is treated as a string.
+      && vars.i == "12345"
+      && vars.j == "54321"
+      // If the expanded value is to be passed as a string, enclose it in double quotes.
+      && vars.k == "123459"
+      && vars.l == "543219"
diff --git a/testdata/book/include_vars_main.yml b/testdata/book/include_vars_main.yml
new file mode 100644
index 00000000..b14c65b5
--- /dev/null
+++ b/testdata/book/include_vars_main.yml
@@ -0,0 +1,25 @@
+desc: Include vars (main)
+vars:
+  intNumber: 12345
+  strNumber: "54321"
+steps:
+  -
+    include:
+      path: include_vars_included.yml
+      vars:
+        # If it is a primitive type, the value is passed as it is.
+        a: 12345
+        b: "54321"
+        # If there is a variable expansion, it is reinterpreted in a form without quotes.
+        c: '{{ vars.intNumber }}'
+        d: '{{ vars.strNumber }}'
+        e: '{{ vars.intNumber }}9'
+        f: '{{ vars.strNumber }}9'
+        g: '9{{ vars.intNumber }}'
+        h: '9{{ vars.strNumber }}'
+        # If cast with string(), it is treated as a string.
+        i: '{{ string(vars.intNumber) }}'
+        j: '{{ string(vars.strNumber) }}'
+        # If the expanded value is to be passed as a string, enclose it in double quotes.
+        k: '"{{ vars.intNumber }}9"'
+        l: '"{{ vars.strNumber }}9"'