PURPOSE: To discriminate the duty cycle with simple constitution by constituting the title circuit with a capacitor cutting off a DC component of an input pulse, a multi-level comparator having plural comparators and voltage division resistor groups and a discriminating circuit discriminating the duty cycle based on the output of the comparators.

CONSTITUTION: An input pulse from an input terminal +4 of the duty cycle discriminating circuit 1 is subjected to DC component cut-off by a capacitor C and the input voltage of the multi-level comparator 2 changes corresponding to the duty cycle of the input pulse. For example, when an input pulse with the same voltage level V and different duty cycle as shown in figures A, B is given to the input terminal 4, it is given to one input terminal + of comparators 3a, 3b... as signals different from voltage levels V', V" shown respectively in hatched lines in the figures A, B and an output corresponding to the duty cycle of the input pulse is given to the discrimination circuit 3 from the multi-level comparator 2.