PURPOSE: To prevent destruction of a data interruption of data transmission when a hit or power failure of a power supply is generated by using a backup power supply formed by a power supply backup means.

CONSTITUTION: When a hit or power failure takes place on a power line 30, it is detected by a detection circuit 212 and a switch circuit 213 is switched to the opposite position by the output of the circuit 212 and a non-power failure power supply outputted from a power supply backup circuit 211 is supplied to a power line transmission function circuit 221 of a power line transmission function section 22. When a data to be sent from a terminal equipment 10 via a data transmission line 12 is fed to the circuit 221, the circuit 221 converts the data by the non-power failure power supply from the circuit 211 to a data to be sent to the power line 30 and sends it to the power line 30 via a capacitor 222. Thus, data destruction or data transmission interruption is not caused at the hit or a power failure of a power supply.