It is an object of this invention to provide an AC adaptor capable of achieving downsizing and cost reduction, and having both a constant-voltage/constant-current control function necessary for battery charge, and an optimum current control function for driving of a low-power-consumption electronic apparatus. This AC adaptor includes a first constant-current control device which performs first constant-current control for charging a battery, a second constant-current control device which performs second constant-current control for supplying an electric current necessary to drive an electronic apparatus, and a voltage detecting device which detects the voltage drop of a DC output. If the output voltage becomes lower than a preset value, the second constant-current control for supplying the electric current necessary to drive the electronic apparatus is performed. Since this obviates the need for a large-capacity element which permits high power, it is possible to decrease the outer dimensions, and also decrease the power ratings of electronic parts.