PURPOSE: To discriminate the rotational speed and the rotary direction of a rotor by detecting a mark on the rotor with two sensors.

CONSTITUTION: M1, M2... and Mn indicate slit groups provided on a rotary disc 5 and desired adjacent two slit rotary discs 5 are at an angle of 360°/n to the center O. Two sensors A and B are arranged at the fixed position near the rotary disc 5 and they A and B are at an angle of θAB<360°/n-θAB to center O near the rotary disc 5. It can be discriminated that the rotary disc 5 turns clockwise when the time tAB between the arrivals of slits at the sensor A and at the sensor B< the time tBA between the arrivals of the slits at the sensor B and at the sensor A, while when tAB>tAB, the rotary disc 5 runs counterclockwise.