DC power supply unit with battery charging function

A DC power source unit supplies a DC voltage of an electrically driven tool via an adapter to and charges a battery pack, which is used as an alternative energy source for the tool. To the DC power source unit of the invention, the charging of the battery block is immediately stopped and the supply of power to the tool immediately started when the power switch of the tool is turned ON. The DC power source unit is also able to selectively output a different drive voltages corresponding to the rated voltages of a plurality of different tools and able to charge battery packs having different battery voltages.