PURPOSE: To obtain mechanical strength, attain high division, and to improve stability in size by arranging a plane part and a V groove part alternately in the outer peripheral part of the rotary disk made of transparent resin.

CONSTITUTION: A plane part 2 and a V groove part 3 are formed alternately at the outer peripheral part of the rotary disk 1 made of transparent resin. Incident light from a light emission part travels straight in a plane part, but is refracted at a V groove part 3 and does not reach a photodetection part. This disk 1 having transmission and nontransmission parts is rotated between a light emitting and a photodetecting element to detect the rotating speed. Thus, a plane part 2 and a V groove part 3 are provided to the outer peripheral part of the disk 1 to constitute a transmission and a nontransmission part, so the problem of a burr treatment and a run of resin during resin formation is eliminated to increase the mechanical strength, attain high division, and improve the stability in size.