PURPOSE: To speed up period difference detection and phase difference detection between two inputs with simple constitution by inputting the counted value of a counter, which is set in response to the input of each pulse of a 1st input pulse train signal, before setting to a register with a 1st input pulse train signal and taking it out as a series signal.

CONSTITUTION: The period comparing circuit is provided with the counter (a) which is set to a specific value in response to the input of each pulse of the 1st input pulse train signal and counts up from the specific value according to a 2nd input pulse train signal in the input period of each pulse, the register (b) which holds and outputs the parallel output counted value of the counter (a) before the counter is set to the specific value in response to the input of each pulse of the 1st input pulse train signal, and a parallel-series converter (c) which converts the parallel output signal of the register into a series signal. Consequently, the period difference detection and phase difference detection between the two inputs are performed speedily with the simple constitution.