PURPOSE: To achieve back-up with a small amount of battery and miniaturization and low cost by supplying power from a backup battery only when a data processing device performs communication processing.

CONSTITUTION: A power supply signal (a) and a charging signal (b) are sent from a power supply 1. When data communication is initiated, a backup request signal (c) starts to be transmitted from a communication control circuit 3 to a battery backup control circuit 2, a power supply signal (d) starts to be transmitted from the battery back-up control circuit 2 accordingly, and then power is supplied from the power supply circuit 1 and the battery backup control circuit 2. When power fails, the communication control circuit 3 receives power only from the battery back-up control circuit 2. When data communication ends, transmission of the power supply signal (d) from the battery back-up control circuit 2 to the communication control circuit 3 is suspended.