PURPOSE: To provide a small-sized hydraulic actuator which converts the fluid pressure to the revolution force and secures the high productivity and has at least four point stop positions.

CONSTITUTION: A hydraulic actuator is equipped with a planetary gear device 21 and the first and second direct operating type hydraulic actuators 24 and 25 for driving a rotary member 21f which rotatably supports the planetary gear 21a and one differential input shaft 21d. Four different angular positions can be obtained by the action of a differential gear mechanism by driving the first and second direct operating hydraulic actuators 24 and 25 at two point positions.