PURPOSE: To cause the exhibition of a fine control resolving power by disposing a reduction gear between the stepping motor and the servo valve in the rotary hydraulic actuator to which the operating angle of the stepping motor is inputted while being controlled.

CONSTITUTION: In response to a signal from a controller 2, a stepping motor 1 drives a sun roller 4 of a planetary roller type reduction gear 3 through a specified angle. This causes the rotation of three planetary rollers 5 circumscribed on the sun roller 4. The planetary rollers 5 cause pins 6 to make their self-rotations around an output shaft 8 of the actuator. As a result, a ring 9 is rotated and, due to the cam action thereof, a spool 11 of a servo valve 10 is displaced. This permits an oil pressure to be supplied into a chamber (f) of a hydraulic piston 13, so that a hydraulic port (d) of low-pressure side is opened to permit discharge of the oil pressure in a chamber (e). This causes the hydraulic piston 13 to be displaced to cause rotation of the output shaft 8. This causes the pins 6 to make their rotations around the planetary roller 5 via a crank 14. By the resulting rotation of the ring 9, the spool 11 is returned to its original position to stop the displacement of the hydraulic piston 13.