A double-ended needle and a mixing instrument which are capable of suppressing a reverse flow of a solution are provided. A double-ended needle (20a) which constitutes part of a mixing instrument (10) includes a first puncture portion (70a) and a second puncture portion (72a) having inner cavities thereof communicating with each other, an inner cavity (76) of the first puncture portion (70a) and an inner cavity (77) of the second puncture portion (72a) communicate with each other, and the lateral cross-sectional area of the inner cavity (76) of the first puncture portion (70a) is smaller than the lateral cross-sectional area of the inner cavity (77) of the second puncture portion (72a).
