A DC power source unit supplies a DC voltage to an electrically powered tool through an adapter and also charges a battery pack used as an alternative power source of the tool. With the DC power source unit of the invention, charging of the battery pack is stopped and supply of power to the tool is started instantaneously when the power switch of the tool is turned ON. The DC power source unit is also capable of outputting selective one of different drive voltages corresponding to the rated drive voltages of a variety of different tools and capable of charging battery packs with different battery voltages.