A power source unit is constructed by an AC/DC switching power source circuit 12, a battery unit 16, switching circuits 13 and 15, detecting circuits 18 and 21 corresponding to the switching circuits 13 and 15, and stop signal forming circuits 19 and 20. In the supplying mode, the power source circuit 12 is made operative, the switching circuit 13 is turned on, an output voltage formed by the power source circuit 12 is supplied to the load side, and at the same time, the switching circuit 15 is turned off by a stop output from the stop signal forming circuit 20. In the charging mode, the power source circuit 12 is made operative, the switching circuit 15 is turned on, the output voltage formed by the power source circuit 12 is supplied to the battery unit 16, and at the same time, the switching circuit 13 is turned off by a stop output of the stop signal forming circuit 19.