PURPOSE: To select the opening/closing angle of a door body and enable door closing function to work at the normal time and stop at the necessary time by providing a stopper for impeding the return of a piston by a spring, in such a way as to be able to adjust the protruding length of the stopper in the moving direction of the piston.

CONSTITUTION: A bar like stopper 9 for impeding the return of a piston 5 by a spring 4 is provided at the end part on the opposite side to the installed side of a screw tap 8 in a piston housing 6. In this case, the stopper 9 is formed of a screw, and one end is extended out of the housing 6, piercing the side wall of the housing 6 so as to fit a rotating grip 10. The grip 10 is rotated to adjust the protruding length of the stopper 9 in the moving direction of the piston 5. The stopper 9 comes in contact with the end of the piston going to return by the spring 4 so as to impede its motion and leave a door body, going to close from that time, open. The optional opening/closing angle of the door body can be thereby selected, and normally door closing function works without fail. The door closing function can be stopped only at the necessary time such as the load carrying or cleaning time.