To provide a small rotary actuator capable of rough rotation control when driving a driving object.

A ring gear 50 has an internal tooth 51 at an internal edge and is fixed so as to be coaxial to an input shaft. A sun gear 60 has a columnar projection 61 projecting from one face in a plate thickness direction, and an external tooth 62 that is engaged with the internal tooth 51, is provided eccentrically in a relatively rotatable manner with respect to the input shaft, and rotates and revolves when the input shaft rotates. An output shaft 70 has a disk 72 where a hole 73 into which the projection 61 can go is formed, wherein a rotation component of the sun gear 70 is transferred when an inner wall of the disk 72 with the hole 73 formed is pushed against an outer wall of the projection 61. The hole 73 is formed in the shape of a circle long in a predetermined direction such that the external shape of the hole 73 almost matches with an outer edge of a locus when a virtual circle 100 having a diameter larger than the external diameter of the projection 61 is subjected to a prescribed amount of movement in the circumferential direction of the disk 72.