PURPOSE: To attain the signal transmission to plural signal transmitting paths without decreasing the amplification degree of an amplifier output signal by prodiving a transistor(TR) operated to ON state or OFF state.

CONSTITUTION: The input impedance of a TRQ11 viewed from an output terminal Tout becomes a comparatively high value by providing the TRQ11 to an interface circuit 2. Thus, almost no current flows from a collector of a TRQ6 of an amplifier circuit 1 to the interface circuit 12. Then, it is not required especially to use TRs having a large power capacity for TRs Q6, Q8 constituting an output circuit, nor an output voltage Vout is attenuated. Thus, the signal transmission to plural signal transmitting paths is attained.