A cordless telephone system comprises a radio telephone set and a connection unit connected to a wire telephone circuit and also connected to the radio telephone set through a radio communication circuit, for interconnection control between the wire telephone circuit and the radio telephone set. When the wire telephone circuit is put in an unusable state or a signal on the radio communication circuit is put in an unreceivable state, the connection unit transmits a disconnecting signal to the wireless telephone set to forcibly cut off the radio communication circuit.