A gear meshing with a pinion controls the angle of rotation of a valve element which is brought into sliding contact with a valve seat, thereby controlling the opening and closing of a communicating hole of outflow pipe. During welding the valve seat plate and a housing, a hook portion of an upper portion of the gear is retained by a retaining portion to cause the valve element to be spaced apart from the valve seat. After natural cooling, the pinion is remotely operated by an electric motor to rotate the gear, thereby canceling the holding. A resilient arm causes the gear to be disengaged from the retaining portion by its urging force, brings the valve element into pressure contact with the valve seat, and allows the opening/closing operation to function smoothly.