PURPOSE: To make constitution comparatively simple and increase rotary torque gradually.

CONSTITUTION: An actuator is equipped with a cylinder 20; a piston 30 which is reciprocatable in the state of being sealed in the cylinder 20 and is arranged so as not to be able to be rotated; a rotary member 40 which is arranged at one end of the cylinder 20 inside face to face with the piston 30 and rotatably and non-reciprocatably; an elastic push-pressing member 46 to energize the piston 30 and the rotary member 40 in directions to separate them from each other at all times; and at least one oblique axis rod 50 to whose both ends the piston 30 and the rotary member 40 are respectively connected so as to be able to move incliningly.