A rotary actuator includes a cup-shaped piston which is movable back and forth and rotatable on its axis. A driving shaft has on its circumferential face a threaded area engaging with a threaded internal face of the cup-shaped piston. The driving shaft can be rotated on its axis during rectilinear motion of the cup-shaped piston. A rotary member has an area engaging with an outer portion of the cup-shaped piston. A casing accommodates the cup-shaped piston, the driving shaft, and the rotary member. The casing has a slit which can be tightened for securing the rotary member to the casing. A regulating member limits the length of strokes of the cup-shaped piston.