To provide an angular acceleration detector which capable of obtaining superior S/N signal by using the diffraction interference figures of light by a three-grating-optical system.

The rotating duck of a three-grating reflex rotary encoder is formed of the first rotating disc 1 on a circumferential surface close to a rotation axis 5, the second rotating disc 2 on a circumferential surface farther from the first one and second rotating disc supporting parts 4 to support the both in between them. A light source 13 produces light, and the light beams through the openings of a grating 10 are reflected at gratings 6 and 7 on each of the disks 1 and 2, respectively. Then angular acceleration is obtained from the phase difference of the light beams received at each of the light-receiving elements 111, 112, 121 and 122.