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 which constitutes part of a mixing instrument includes a first puncture portion and a second puncture portion having inner cavities thereof communicating with each other, an inner cavity of the first puncture portion and an inner cavity of the second puncture portion communicate with each other, and the lateral cross-sectional area of the inner cavity of the first puncture portion is smaller than the lateral cross-sectional area of the inner cavity of the second puncture portion.
