A rotation actuator is equipped with a motor and a reduction gear disposed adjacent to each other. Both shaft end portions of a rotating shaft, which passes through the center of the motor and reducer, are rotatably supported through first and second rolling bearings by first and second fixed discs that are mutually fastened and fixed. A rotation output plate is supported through a third rolling bearing in the outer peripheral surface of the second fixed disc. A sliding ring is mounted between the rotation output plate and the first fixed disc. The sliding ring functions as a thrust bearing for the rotation output plate and also functions as a sealing mechanism for preventing grease from flowing from the reducer gear to the outside. A flat rotation actuator can be achieved.