A piston (4) is inserted in a housing (1) so that the piston (4) is vertically movable. A housing hole (5) is vertically provided in the piston (4), and an output rod (6) is inserted in the housing hole (5) so that the output rod (6) is vertically movable. A converting mechanism (22) converts vertical movement of the piston (4) into rotary movement of the output rod (6). A guide groove (28) is provided in a circumferential direction on an inner circumferential wall of the housing hole (5), and a stopping part (29) is provided at an end part, in the circumferential direction, of the guide groove (28). An engaging member (31) which is provided on an outer circumferential wall of the output rod (6) is caused to face the stopping part (29) of the guide groove (28) at a given distance in the circumferential direction from the stopping part (29) so that the engaging member (31) can be in contact with the stopping part (29).