PURPOSE: To attain a battery exchange without disconnecting a communication line even when the output voltage of a portable telephone terminal drops while being talking over the telephone between the portable telephone terminal and the other telephone terminal.

CONSTITUTION: When a main battery output voltage monitoring circuit 3 of a portable telephone terminal 1 detects the drop of the output voltage of a main battery, an automatic power source switching circuit 6 switches the main battery to an auxiliary battery 9, and a control part 5 communicates the information of the drop of the output voltage of the main battery to a mobile telephone switchboard 14. The mobile telephone switchboard 14 receives the information from the portable telephone set 1, communicates the battery exchange to the other side subscriber, and holds the line of contact in a fixed time regardless of a radio carrier state. Thus, the main battery can be exchanged while the communication is maintained.