A reagent vessel cap includes an elastic sealing member having radial slits from the center toward the outer periphery, the sealing member being arranged on the opening of a vessel body supported by a cap body, for shielding a reagent contained in the vessel from outside air; and a pressurizing member attached to the cap body above the sealing member and retained in position by the biasing force of a built-in spring member, for opening the sealing member downward along the slits by the pressure from above and returning to position by relaxing of the pressure.