To decrease power consumption at the time of turning off the power source of a connected communication equipment, and to maintain the communicating function for management of the device itself in this state.

A line monitor circuit 22 detects whether a line between a controller 14 and a personal computer 20 is valid (a link state) or invalid (no- link state). When the line is valid, an on/off switch 23 is turned on through a microprocessor 13, and a power supply voltage V2 is supplied from a power source circuit 21 to a digital modulator 11 and a digital demodulator 12, and when the line is invalid, the on/off switch 23 is turned off through the microprocessor 13, and the supply of the power supply voltage V2 from the power source circuit 21 to the digital modulator 11 and the digital demodulator 12 is stopped.