PURPOSE: To attain the common use of a power transmission line and a signal line by inverting the polarity of a power supply connecting to a power supply line in response to the sent signal in the case of supplying power.

CONSTITUTION: The power supply for other device built in one device is not supplied to other device via a power line or the like but the polarity of the power supply is inverted by a polarity inversion circuit in response to the sent signal and sent via a transmission line L. The other device supplied from one device via the transmission line L uses a rectifier section 10 to rectify the power and the supply is used for the power supply of the other device and a signal detection section 11 detects the polarity of the supplied power to extract the content of the signal. In the case of supply power, the polarity of the power supply connecting to the power supply line in response to the sent signal is inverted to use the power supply line and the signal line in common.