doujini-vs-isseini

同時に (doujini) and 一斉に (isseini) both translate as “at the same time” in English — which is exactly why they confuse learners. The difference comes down to one key question: are we talking about two things happening simultaneously, or about a group doing something all at once? 同時に is for simultaneous events (two or more things happening at the same time). 一斉に is specifically for a group acting together in unison. This guide explains the distinction with clear examples.

Yuka

Rei, my teacher mentioned 同時に and 一斉に today. What’s the difference?

Rei

Great question! Both come up a lot in everyday Japanese. The trick is knowing which context calls for which — let me show you!

At a Glance: 同時に vs. 一斉に

同時に (doujini) — “At the Same Time” / Simultaneous Events

同時に is the general-purpose adverb for “at the same time.” It simply states that two (or more) things happen at the exact same moment. The subjects can be the same person, different people, or even unrelated events. It does not require a large group.

同時に also appears in the pattern ～と同時に to express “at the same time as ___” — useful for describing when two life events coincide, or when someone has two simultaneous roles.

Example 1 — two events at the same moment:

ゆかとたつやは同時に話し始めた。
Yuka to Tatsuya wa doujini hanashi hajimeta.
Yuka and Tatsuya started talking at the same time.

Example 2 — two life events coinciding:

卒業と同時に結婚した。
Sotsugyou to doujini kekkon shita.
I got married at the same time as graduating.

Example 3 — dual role / description:

彼は優秀な選手であると同時に、チームのリーダーでもある。
Kare wa yuushuu na senshu de aru to doujini, chiimu no riidaa demo aru.
He is an excellent player and at the same time also the team leader.

Yuka

I see… so context really matters with 同時に? It’s not just about the literal meaning?

Rei

Right! Japanese often works that way. 同時に especially has nuances that go beyond a direct translation — keep that in mind.

一斉に (isseini) — “All at Once” / The Group Acts Together

一斉に specifically emphasizes that many people (or things) all do the same action together at the same moment. The key nuance is uniformity and collective action — not just that things happen to coincide, but that a group moves as one. It carries a slightly dramatic or ceremonial feeling, like the moment of a starting pistol or everyone applauding together.

一斉に requires a plural or group subject. You would sound odd using 一斉に to describe just two people unless they are explicitly acting as a synchronized pair.

Example 1 — group action:

スタートの合図と共に、選手たちは一斉に走り出した。
Sutaato no aizu to tomo ni, senshu-tachi wa isseini hashiridashita.
At the starting signal, all the athletes started running at once.

Example 2 — collective reaction:

観客は一斉に拍手した。
Kankyaku wa isseini hakushu shita.
The audience all applauded at once.

Example 3 — simultaneous collective action in nature / events:

桜の花が一斉に咲き始めた。
Sakura no hana ga isseini saki hajimeta.
The cherry blossoms all began blooming at once.

Yuka

Got it. And 一斉に — is that the opposite, or more like a different usage?

Rei

More of a different usage! 一斉に carries its own feel. Comparing them together like this is actually the fastest way to master both.

The One Key Difference to Remember

Decision Flowchart: 同時に or 一斉に?

Are you describing things happening at the same time?
                    |
                    v
Is the subject a GROUP (many people / things)
doing the SAME action in UNISON?
         |                    |
        YES                   NO
         |                    |
         v                    v
      一斉に              Are it just 2 things /
  (group unison,         people at the same moment?
   collective action)         |              |
                             YES             (dual role / coinciding events)
                              |              |
                              v              v
                           同時に         同時に
                         (simultaneous)  (～と同時に pattern)

Quick Quiz — Test Yourself!

Yuka

I think I’ve finally got it! Let’s take the quiz to be sure.

Rei

Perfect confidence check! Let’s go — you’ve got this!

Choose 同時に or 一斉に for each sentence.

Q1. The whole audience started clapping all at once.
観客が＿＿＿拍手し始めた。
Kankyaku ga ___ hakushu shi hajimeta.

Answer: 一斉に (isseini)
Reason: A large group (観客) acting in collective unison — 一斉に is the natural choice.

Q2. She became a mother and a business owner at the same time.
彼女は母親になると＿＿＿、事業主にもなった。
Kanojo wa hahaoya ni naru to ___, jigyounushi ni mo natta.

Answer: 同時に (doujini) — ～と同時に
Reason: Two life roles coinciding for one person — the ～と同時に pattern uses 同時に.

Q3. At the signal, all the runners started at once.
合図と共に、ランナーたちが＿＿＿スタートした。
Aizu to tomo ni, rannaa-tachi ga ___ sutaato shita.

Answer: 一斉に (isseini)
Reason: A group (ランナーたち) acting in synchronized unison — 一斉に is most natural here.

Q4. Both of them said “I’m sorry” at exactly the same moment.
二人は＿＿＿「ごめんなさい」と言った。
Futari wa ___ “gomen nasai” to itta.

Answer: 同時に (doujini)
Reason: Only two people coincidentally saying the same thing at the same moment — 同時に is the natural choice for small numbers.

Q5. All the lights in the stadium went on at once.
スタジアムのライトが＿＿＿ついた。
Sutajiamu no raito ga ___ tsuita.

Answer: 一斉に (isseini)
Reason: Many lights all switching on collectively — 一斉に emphasizes the “all of them together” nuance.

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あわせて読みたい

Now that you understand 同時に and 一斉に, explore ながら vs. 同時に — which covers how to use 同時に in contrast with the ながら conjunction:

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