PURPOSE: To detect the rotational state of a machine easily with no influence of noise even in such a place as being subjected to significant electromagnetic noise or the earth potential is unstable.

CONSTITUTION: A two-phase step motor M is driven by a generator and the power induced in the stator windings 1, 2, 5, 6 of respective phases of the motor M is stabilized through constant voltage circuits 3, 7 and constant current circuits 4, 8 before being fed to LEDs 10, 12. Optical pulse signals from the LEDs 10, 12 are transmitted on optical fibers 13, 14 to photodiodes 15, 16 where the optical pulse signals are converted into electric signals. Rotational speed, rotational angle, angular direction, etc., are detected by measuring electric signals from the photodiodes 15, 16. Since no external power supply is not required for the generation of the optical pulse signal, no electromagnetic noise intrude from a feeder line.