たとたん | The Moment That in Japanese

Instantly, and Unexpectedly

たとたん says the second thing happened the instant the first did — and that it took the speaker by surprise.

These are what the pattern is really about, and breaking any one of them makes the sentence wrong:

Past tense only. The event has already happened. There is no future たとたん.
No intention in the second clause. It must be something that simply occurred — not a plan, a request or a decision.
Surprise. If the outcome was expected, use てから or と.


○ 開(あ)けたとたん猫(ねこ)が逃(に)げた	unplanned, surprising
× 開(あ)けたとたん入(はい)りましょう	a suggestion

と also links an action to an immediate result, but it accepts habitual and expected pairings. たとたん is for the single, startling moment.

The に is optional and often present in writing. Meaning is unchanged.

Example Sentences

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Speed

立(た)ち上(あ)がったとたん、めまいがした。

The moment I stood up, I felt dizzy.

ドアを開(あ)けたとたん猫(ねこ)が逃(に)げ出(だ)した。

The instant I opened the door, the cat bolted.

外(そと)に出(で)たとたん雨(あめ)が降(ふ)り出(だ)した。

No sooner had I gone out than it started raining.

座(すわ)ったとたんに電話(でんわ)が鳴(な)った。

The phone rang the second I sat down.

顔(かお)を見(み)たとたん涙(なみだ)が出(で)た。

The moment I saw her face, I started crying.

薬(くすり)を飲(の)んだとたん眠(ねむ)くなった。

I got sleepy the instant I took the medicine.

電車(でんしゃ)を降(お)りたとたん財布(さいふ)を落(お)としたことに気(き)づいた。

The moment I got off the train I realised I'd dropped my wallet.

春(はる)になったとたん暖(あたた)かくなった。

It turned warm the moment spring arrived.