5 by changing on off ratio d 2 of first switching element 11 2 selector switch 17 is eliminated and switch 31 is provided so that the switch is connected in series with second switching element 13 and switched on when power is supplied to load 5 the on off control of switch 31 is made by control circuit 15 3 third switching element 33 is provided so that the switching element is connected between first switching element 11 and the negative terminal of dc voltage source 1 repeats switching on off alternately with first switching element 11 and is normally switched off when power is supplied to load 5 the structure other than the above is identical with that of the first exemplary embodiment next a description is provided of the operation of the power supply device structured as above first when the ignition switch not shown of a vehicle is turned on the external ecu transmits a charging signal to control circuit 15 so that auxiliary voltage source 3 is charged in response to this signal control circuit 15 switches auxiliary voltage source selector switch 19 to the charge side and turns off switch 31 thereafter control circuit 15 controls first switching element 11 and third switching element 33 so that one of the first and third switching elements is alternately switched on thereby in a manner similar to the first exemplary embodiment two way dc dc converter 29 starts to charge auxiliary voltage source 3 using the power of dc voltage source 1 when the charging makes voltage vc of auxiliary voltage source 3 equal.