A reagent vessel cap includes a sealing member for sealing the opening of a vessel body; an pressurizing member linked to a retaining part for sealing the sealing member, for bringing the sealing member in close contact with the opening all the time, wherein when pressure is applied, the pressurizing member lifts the sealing member against the biasing force of itself to open the vessel and, when the pressure is eliminated, the pressurizing member returns to position by the biasing force to close the vessel by the sealing member; and a cap body capable of mounting the sealing member and the pressurizing member to the opening of the vessel body containing a reagent. The reagent is collected using the cap structure.