diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 4dd9b6ad0..473e92b50 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -44,3 +44,15 @@ Stands for _**S**ervice**W**orker_.
 
 #### Denyaize
 Nyaizeを解除すること
+
+## Code style
+### Don't use `export default`
+Bad:
+``` ts
+export default function(foo: string): string {
+```
+
+Good:
+``` ts
+export function something(foo: string): string {
+```