A PCR vessel (1) is mainly constituted of a vessel body (11) having a reaction chamber (12) storing and reacting a specimen and a reagent at the time of PCR on a bottom portion (14) thereof, a lid body (6) fittable with the vessel body (11), and a reagent cassette (16) stored in a portion between a lower surface (7) of the lid body (5) and the reaction chamber (12) in the vessel body (11). The reagent cassette (16) has at least one reagent storing portion capable of sealing the reagent with sealing means such as paraffin solid before PCR and melted by heating in PCR. When structuring the PCR vessel in this manner, the sealing means is melted by heating in PCR and the reagent storing portion is opened when storing a specimen necessary for PCR in the vessel body and performing PCR in a state previously storing and sealing the reagent in the reagent storing portion. Then, the specimen and the reagent move to the lower reaction chamber.