PURPOSE: To make the center frequency of a filter variable stably by constituting this filter by using a N path filter where the center frequency is determined only by a clock frequency.

CONSTITUTION: Signals supplied to input terminal 1 are supplied to amplifier 3 through resistor 2, and are amplified, and have the phase inverted and appear at output terminal 8. Signals, which pass through N path filter 4, of these output signals are supplied to amplifier 3 again as negative feedback signals. Consequently, the feedback quantity is increased only in respect to frequencies which can pass through N path filter 4, and on appearance, the amplification degree of amplifier 3 is lowered, so that the characteristic as a notch filter can be obtained. As a result, the N path filter is used to change the center frequency by change of the clock frequency, and specific frequency components can be eliminated stably.