Implement findstring
diff --git a/func.go b/func.go
index ff09650..7ada6ba 100644
--- a/func.go
+++ b/func.go
@@ -46,7 +46,14 @@
 }
 
 func funcFindstring(ev *Evaluator, args []string) string {
-	// TODO(hamaji): Implement
+	if len(args) < 2 {
+		panic(fmt.Sprintf("*** insufficient number of arguments (%d) to function `findstring'.", len(args)))
+	}
+	f := ev.evalExpr(args[0])
+	text := ev.evalExpr(strings.Join(args[1:], ","))
+	if strings.Index(text, f) >= 0 {
+		return f
+	}
 	return ""
 }
 
diff --git a/test/findstring.mk b/test/findstring.mk
new file mode 100644
index 0000000..5f53ad9
--- /dev/null
+++ b/test/findstring.mk
@@ -0,0 +1,6 @@
+test:
+	echo $(findstring a, a b c)
+	echo $(findstring b, a b c)
+	echo $(findstring b c, a b c)
+	echo $(findstring a, b c)
+	echo $(findstring a, b c, a)