A reagent vessel is mounted for swinging movement and has a projection at the bottom portion thereof. A rotor has an eccentric groove at an upper end and has an upper sealing member and a lower sealing member. A reagent refrigerator has a sealing at a bottom portion. The rotor is driven by a motor installed outside the refrigerator which moves up and down. When the rotor is disposed at a top dead point, the groove of the rotor engages with the projection of the vessel, then the rotor rotates and the reagent is mixed in swinging rotation. In this state, the lower sealing member of the rotor seals a bottom face of the sealing member of the refrigerator and prevents the water droplets falling down from the refrigerator from falling into the motor. When the rotor is disposed at a bottom dead point, the groove of the rotor disengages from the projection of the vessel. In this state, the upper sealing member of the rotor contacts an upper face of the sealing member of the refrigerator and the refrigerator is sealed.