PURPOSE: Not to generate a mispulse even in case when a generator has rotated from stop or a holding value has dropped due to a low speed, by providing the minimum value of an absoulte value of a peak value of a fundamental wave, in a pulse generator for detecting a rotation.

CONSTITUTION: A mispulse is prevented by detecting a transmission light beam by a photoelectric detector by cutting a slit on a disk, holding the maximum value and the minimum value in a positive side circuit 3 and a negative side circuit 4 of the sampling and holding circuit, respectively, generating a reference in accordance with the number of pulse by providing said values across a voltage dividing resistor 7, outputting a timing pulse by which a signal of the photoelectric detector passes through the reference, also applying the voltage across the voltage dividing resistor 7 through a resistor 8 by a positive electric power source and through a resistor 9 by a negative electric power source, and constituting so that a voltage difference across the voltage dividing resistor 7 does not become below the voltage desided by the resistor 8, the voltage dividing resistor 7 and the low resistor 9, even if a holding value of the sampling and holding circuit becomes zero.