A three-position stop type swing actuator has a main piston freely moving in forward and backward directions in an axial direction within a casing, an output shaft placed at a coaxial position with the main piston in such a manner as to be fixed in an axial direction and freely rotate around an axis, a conversion and transmission mechanism converting a forward and backward motion of the main piston into a rotating and swinging motion of the output shaft, and a sub piston having a stroke smaller than that of the main piston a fluid pressure operating force larger than that of the main piston and arranged within the casing in such a manner as to freely move in the forward and backward directions and be capable of being brought into contact with the main piston so as to restrict a middle stop position of the main piston.