JLPT N3Spoken and writtenExact timing

と同どう時じにat the same moment; as well as being

Two things happen at the same moment: the second starts right as the first does. It also says that two things are true of one person or thing at once.

the same moment

ドアが開ひらくと同どう時じに、子こどもたちが外そとへ飛とび出だした。

The moment the door opened, the children rushed outside.

two things at once

この建たて物ものは図と書しょ館かんであると同どう時じに、地ち域いきの交こう流りゅう拠きょ点てんでもある。

This building is a library and also a meeting place for the community.

The same moment

Verb dictionary form／noun + と同どう時じに

The verb usually stays in the dictionary form even when the sentence is about the past.

Two things at once

Noun／な-adjective + である + と同どう時じに

An い-adjective needs no である: うれしいと同どう時じに.

Before it: Mostly something that happens at one point: 開ひらく, 着つく, 終おわる, 卒そつ業ぎょう, 開かい店てん, 発はつ売ばい.
After it: What happens at that same moment, planned or not.
Tone: Neutral and exact. There is no surprise in it.
Where you'll meet it: News, stories, instructions, and conversation when the timing matters.

In use

卒そつ業ぎょうと同どう時じに、地じ元もとを離はなれた。

I left my hometown the moment I graduated.

列れっ車しゃが駅えきに着つくと同どう時じに、雨あめが降ふり始はじめた。

It started to rain just as the train pulled into the station.

新しん製せい品ひんの発はつ売ばいと同どう時じに、公こう式しきウェブサイトも公こう開かいされた。

The official website went live at the same time as the new product went on sale.

Two things at once

With である or an adjective in front, nothing happens at a moment. You are saying that two things are true together, and the second usually carries も.

娘むすめの結けっ婚こんはうれしいと同どう時じに、少すこし寂さびしくもある。

My daughter is getting married. I'm happy, and at the same time a little sad.

Close neighbours

たとたんN3

Also "the moment", after the た-form, but what follows is sudden and nobody planned it. A request can't follow たとたん. After と同どう時じに it can.

ドアを開あけたとたん、冷つめたい風かぜが吹ふき込こんできた。

The moment I opened the door, a cold wind rushed in.

たら、すぐN4

The everyday "as soon as": one thing is done, and the next follows straight away. と同どう時じに is for the very same moment.

駅えきに着ついたら、すぐ電でん話わしてください。

Please call as soon as you arrive at the station.

とともにN3

Stiffer, and mostly for "together with" a person or for change over time.

技ぎ術じゅつの進しん歩ぽとともに、製せい品ひんの性せい能のうも向こう上じょうした。

As technology advanced, the products performed better too.

Check yourself

1A teacher is explaining how the test will start. Which one fits?

ベルが（　　）、始はじめてください。

Choose an answer to see why.

2What does this say about him?

彼かれは医い者しゃであると同どう時じに、作さっ家かでもある。

Choose an answer to see why.

More detail

With no と, 同どう時じに is the plain adverb "at the same time".

二人ふたりは同どう時じに手てを挙あげた。

The two of them put their hands up at the same time.

More sentences

会かい議ぎが終おわると同どう時じに、参さん加か者しゃたちは席せきを立たった。

The moment the meeting ended, the participants got up from their seats.

新あたらしいシステムを稼か働どうさせると同どう時じに、旧きゅうシステムを停てい止しした。

We shut down the old system at the same time as we started up the new one.

社しゃ長ちょうが壇だん上じょうに上あがると同どう時じに、会かい場じょうから拍はく手しゅが起おこった。

As the company president stepped onto the stage, applause broke out in the hall.

朝あさ日ひが昇のぼると同どう時じに、霧きりが薄うすれ始はじめた。

As the sun came up, the mist began to thin.