to burst into tears; to burst out crying; to begin to cry; to be moved to tears
赤ちゃんが泣き出した。
The baby burst into tears.
急に泣き出すので周りが驚いた。
Everyone was surprised when I suddenly started crying.
感動的な場面を見て思わず泣き出してしまった。
I saw a touching scene and couldn't help but start crying.