PURPOSE: To control variably the cut-off frequency over a wide range by using a variable resistance element to the resonator which is set opposite to a capacitor that decides the cut-off frequency and supplying a control current to the variable resistance element.

CONSTITUTION: When a current I1 flows to a variable resistance element R1, the potential at a point b1 tries to have an accurate shift by (I1×R1) with no shift given to the potential at a point a1 of the inflow side. In this case, however, a local loop of a control circuit consisting of a comparator, a photocoupler, etc. is actuated to set the potential Eb1 at the point b1 at a level equal to the DC reference voltage E1. Then the effective resistance R1 of the variable resistance element is equal to E1/I1 by the current I1 via a double negative feedback circuit consisting of a loop circuit of a filter signal line and an added local loop circuit. So is with a variable resistance element R2 with the cut-off frequency set at 1/2πCR.