PURPOSE: To eliminate maintenance of a power supply at a terminal unit for driving a secondary cell by notifying detection information to a power supply device when the output voltage drop of the cell of the terminal unit is detected and charging the cell via the information channel of a home bus.

CONSTITUTION: A terminal unit 6 in a portable type terminal 3 and an interface unit IFU 4 are driven by a secondary cell 5. When the output voltage of the cell 5 drops, it is detected by a cell voltage monitor 9, and notified to the unit 6. The unit 6 notifies a power transmission request to a power supply 10 via a control channel 2 by using the IFU 4 by the notification. The IFU 4 starts a power supply 12 when it receives the notification of the completion of an information channel connection, and supplies charging power to the cell 5 in the terminal 3 via information channels 1a∼1c.