PURPOSE: To obtain a desired transfer function, to reduce the size of a transmission system circuit, and to simplify the circuit by determining whether input terminals are applied with input signals or a reference potential through the switching of a filter which uses a small number of capacitors and a switch.

CONSTITUTION: Input terminals 1∼3 and one output terminal 4 are provided to a filter circuit FL, which is provided with capacitors C1∼C4, and C, and a switch and an operating device. Then, the circuit FL operates as an LPF by applying a signal to the input terminal V1 with the input terminals V2 and V3 grounded or as a BPF by applying the signal to the input terminal V2 with the input terminls V1 and V3 grounded. It also operates as an HPS by applying the signal to the input terminal V3 with the others grounded. Further, the circuit operates as a BEF by applying signals to the input terminals V1 and V2 with the input terminal V1 grounded, and as a bisecondary filter by applying singals to the input terminals V1∼V3 at the same time. Then, the desired filter circuit is reduced in size to simplify a transmission system circuit.