PURPOSE: To prevent a battery power source from being wasted by supplying power to an NRS detection circuit or a coincidence judge circuit with polarity inversion by utilizing the voltage of a telephone line for a voltage for an internal circuit.

CONSTITUTION: When a polarity inversion detection circuit 1 detects slack polarity inversion, the ON of a power source circuit 2 is controlled, and power down cancel is performed. Thus, an NRS detection circuit 3 is turned to an active state. When an NRS signal can not be detected after the slack polarity inversion or polarity inversion, namely, in the case of the polarity inversion purposing something excepting for communication with a present terminal, after the lapse of fixed time, power clown is performed by a timer circuit inside the NRS detection circuit 3. In this case, since the power source of the NRS detection circuit 3 does not depend on a battery 6, the battery 6 is not consumed at such a time point. On the other hand, when the NRS signal is detected by the NRS detection circuit 3, power down cancel is performed to a control circuit 5 by the NRS detection circuit 3. Then, communication is performed by a transmission circuit 4 under the control of this control circuit 5 later.