PURPOSE: To guarantee the operation of a multi-functional telephone set in the case of a short break or a failure in the commercial power by monitoring the commercial power source and supplying a power source from a secondary battery in the case of a short break or a failure in the commercial power.

CONSTITUTION: A commercial power source monitoring circuit 2 monitors the condition of the commercial power source (an operating, power failure, or short break condition) from the operating condition of a rectifying device 1 and controls a power source switching circuit 3 in correspondence to the condition. When the commercial power source is in the operating condition, the power source switching circuit 3 supplies the output of the rectifying device 1 to a power source receiving circuit 5 and also supplies the output to a secondary battery 4 and charge operation is executed. In the case of a short break or commercial power a failure in the output of the rectifying device 1 is interrupted and the secondary battery 4 is discharged. Then, power feeding is executed from the secondary battery 4 to the power source receiving circuit 5. Thus, even in the case of a short hit or a failure in the commercial power the power source is supplied from the secondary battery and the operation of the multi- functional telephone set can be guaranteed.