PURPOSE: To enable accurate discrimination even with the duty as threshold other than 50%, by counting a time range of H level and that of L level in a signal to be discriminated with counter.

CONSTITUTION: An incremental/decremental counter 1 performs an incremental counting when an signal to be discriminated is at a H level and a decremental counting when it at an L level. An output of a reference clock signal generation circuit 3 is inputted into a 1/2 frequency division circuit 4 and a 1/3 frequency division circuit 5 and a gate circuit 5 selects an output of the frequency division circuit 4 when the signal is at the H level while it selects an output of the frequency division circuit 5 when the signal is at the L level. A D type flip flop 7 is reset when counts obtained with the counter 1 at one cycle of the signal being discriminated are positive while it is set when they are negative. The duty of the threshold can be set as desired by determining the frequency of a clock signal of the increment in the counting of the counter and that of a clock signal of the decrement in counting.