口くち元もとが緩ゆるむ

expression, godan verb (-mu)

smile; crack a smile

Describes the mouth relaxing into a smile, often involuntarily or when one is amused. The subject is usually the person whose mouth relaxes.

• 彼かれの冗じょう談だんを聞きいて、思おもわず口くち元もとが緩ゆるんだ。

  Hearing his joke, I couldn't help but smile.

• 赤あかちゃんを見みていると、自し然ぜんと口くち元もとが緩ゆるむ。

  When I look at the baby, my mouth naturally relaxes into a smile.