PURPOSE: To provide the noise reducing device of simplified constitution which reduces a periodic noise.

CONSTITUTION: The noise reducing device which reduces a noise signal from a microphone 15 by adaptively controlling the tap value of an adaptive filter 4, generating a signal for reducing a noise inputted to the microphone 15, with the output of the microphone 15 is equipped with a synchronizing pulse generating circuit 1 which generates pulses synchronized with the period of the noise, a pulse interval detecting circuit 2 which detects the generation intervals of the pulses generated by the synchronizing pulse generating circuit 1, and a tap quantity switching part 3 which performs switching to the number of taps of the tapped delay line of the adaptive filter 4 so that the pulse intervals become equal to the intervals detected by the pulse interval detecting circuit 2.