A bolt has a cylindrical fit receiving portion. When the bolt slides forward, the rear part of a valve body enters the fit receiving portion. At a closed end opposed to its opening behind the fit receiving portion, a cylindrical body is provided. The cylindrical body lets the interior of the bolt and the exterior of the bolt communicate with each other. The shank of an opening/closing body is slidably placed in the cylindrical body. This opening/closing body has an lid portion at the end of the shank on the fit receiving portion side and a coming-off preventing portion at the outside end of the shank. An opening/closing body spring is placed between the closed end of the bolt and the lid portion and pushes the lid portion toward the fit receiving portion.