PURPOSE: To hold a current value to a telephone circuit in a standard by disconnecting a DC loop circuit of a modem, in the case an option device is connected to the modem, and allowing an internal circuit of the option device to have resistance being equal to that of the DC loop circuit.

CONSTITUTION: In the case an option device 9' is connected to a modem device 1, a connector B' 8 is connected to a connector A' 7, a loop circuit of a telephone circuit - a connector B 4 - a connector A 3 - the connector A' 7 - the connector B' 8 - the option device 9' is formed, and also, a DC loop circuit 9 is disconnected from the telephone circuit. Accordingly, by allowing the impedance of an internal circuit of the option device 9' to conform to a standard of the telephone circuit in advance, a current flowing to the telephone circuit can be allowed to conform to the standard. In this regard, in the case the option device 9 is not connected, a short circuit connector B'' 8' is connected to the connector A' 7, and a standard current is allowed to flow to the telephone circuit.