PURPOSE: To simplify mounting and prevent output fluctuation of an optical detector, by mounting a stationary bench coaxially with a rotating shaft through a bearing provided with light-projector and light-receiver in a peripherally opposed positions to a rotating body.

CONSTITUTION: A stationary bend 11 is mounted through a bearing 14 in such a way that it is coaxial with a rotating shaft 15 and light-projector 12 and light-receiver 13 are installed on this bench 11. These light-projector 12 and light receiver 13 are opposedly positioned along the diametrical direction of the shaft 15 with the specified gap, and a slit mask 18 is set right in front of the light-receiver 13. And, a rotating body 16 provided with a plurality of slits and mounted on the rotating shaft 15 is positioned within above gap. By this arrangement, fluctuation of output characteristics of the optical detector is prevented either for an axial change or for a diametral change.