PURPOSE: To enable the discrimination of the rotating direction using one sensor by comparing intervals of the detection time checking more than two marks provided at specified positions on a rotary circle.

CONSTITUTION: Marks M1, M2, M3 and the like numbering 3 or the like are provided on a rotary circle 1 in such a manner that angles θ12, θ23 and θ31 forming a sector, for instance, meet the requirement of θ12<θ23<θ31 or the like and are detected with a mark sensor 2. Intervals C12, C23 and C33 between mark detections with the sensor 2 are compared sequentially, which reveals that when the circle 1 rotates clockwise, the preceding one is larger than the current one twice while smaller once wherever the mark detected first is any among those M1WM3. On the other hand, when the circle 1 rotates counterclockwise, the preceding one is larger than the current one once while smaller twice. This enables the discrimination of the rotating direction using one sensor.