PURPOSE: To secure a steady filter characteristics over a wide range, by constituting each of the LCR elements with a passive circuit formed by combining a capacitor and a switch plus a circuit that delivers the mean value of the output voltage of the said passive circuit.

CONSTITUTION: The both ends of a capacitor is short-circuited with every period when the pulse charge flows to secure a function of the resistance R for the equivalence or the like. At the same time, a capacitor is combined with two inverse switches to change the direction of the capacitor with every cycle, thus securing a function of an inductance L. In such way, the circuit A is replaced with the LCR element, thus realizing an optional secondary filter function at the circuit A. The voltage delivered to the terminal 3 of the circuit A is supplied to the circuit B. The circuit B consists of the operational amplifier 4, capacitors Cα and Cβ plus a plural number of switches, and then calculates the mean value of the output voltage of the circuit A before and after the pulse current flows to the circuit A to deliver it through the terminal 2. In such constitution, the filter function is decided at a passive circuit to obtain an extremely stable filter characteristics.