How to say "simultaneously" in Japanese

• 一斉にいっせいにCommon
  adverbCovers 'simultaneously' when many people or things do the same thing at the same moment, all together. It is the 'all at once; in unison' word, as when a crowd acts as one.
  人々ひとびとが一斉いっせいに走はしり出だした。↗

  People started to run all at once.
• 同時にどうじにCommon
  expression, adverbThe usual, safest choice for 'at the same time': two or more events or actions happening or done together, concurrently. When unsure which word to use, pick this one.
  二人ふたりの走者そうしゃは同時どうじに決勝けっしょう線せんに着ついた。↗

  The two runners reached the finish line at the same time.
• 一時にいちどきにCommon
  adverbGlossed 'simultaneously' too, but its chief senses are 'at a time; at one time' — doing or handling a number of things in one go. Seldom the word you want for plain 'simultaneously'.
• 中にもなかにもCommon
  expression, adverbCovers 'at the same time' in the 'both' sense: one subject having two qualities or aspects at once. For events simply happening simultaneously, it is seldom the word you want.

Choose 「同時に」 for the plain 'at the same time'; 「一斉に」 when a group acts all at once, in unison; 「一時に」 for dealing with several things at one time; 「中にも」 chiefly for the 'both ... and ...' sense.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

• 一斉にいっせいにsimultaneously
• 同時にどうじにat the same time
• 一時にいちどきにat the same time
• 同体どうたいas one flesh or body
• 時を同じくしてときをおなじくしてat the same time
• その足でそのあしでstraight (from one place to another)
• どちらからともなくboth
• 中にもなかにもat the same time