PURPOSE: To eliminate noise mixed between pulse input signals by using the time width which is obtained by multiplying the detected period of a pulse signal by a coefficient which is larger than 0 and smaller than 1 as a mask time from the signal which is input next to the previously input signal to the signal which is to be input next.

CONSTITUTION: A pulse signal (a1) becomes a signal (b) through a mask circuit 1 and at the same time is fed to a control circuit 2 via a branch point P. Then, when a next pulse signal (a2) is input to the circuit 1 after the lapse of a time T1 from the signal (a1), the control circuit 2 transfers a count value τ1 of an up counter circuit 3 and, at the same time, outputs a mask signal (c) for prohibiting the passage of the pulse signal input to the circuit 1. Then, a multiplication circuit 4 multiplies the count value τ1 input from the circuit 3 by a coefficient x which is larger than 0 and less than 1 to obtain xτ1. Therefore, if there is noise (n) within a time xT1 which corresponds to the value xτ1, the noise (n) is eliminated an the output signal (b) does not include the noise (n).