at once

早速、直ちに、至急、即刻

at once の類語

immediately, right away, now, directly, instantly, without delay, at one time, like a shot, forthwith, straightaway, at a time, straight off, quickly, in no time, then and there, on the run, all together, simultaneously, promptly, in passing, together, as one, away, soon, time, once, right, straight, shot, double, straightway, swiftly, demand, flat, right now, right off, all at once, rapidly, pronto, nail, speedily, in common, with all speed, this minute, this very minute, readily, expeditiously, on the spot, in agreement, apace, outright, jointly, at one jump, mutually, at a blow, at one blow, with a rush, in partnership, on the instant, without further delay

例文

• It is necessary that you start at once.
  君は今すぐ出発せねばならない。
• You must apologize to her, and that at once.
  君は彼女に謝らなければならぬ、しかもいますぐに。
• You may as well leave at once.
  君は直に出発する方がいい。

at once の言い換えを探すには

at once（早速）の類語・言い換え（パラフレーズ）を、日本語訳つきで関連度順に並べています。英作文の語彙を増やしたいとき、長文読解で本文と選択肢の言い換えを見抜きたいときに使えます。