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, a microcomputer receives a temperature signal indicative of a temperature of the battery pack and determines that the battery pack is fully charged based on the temperature signal. This determination is made regardless of whether the battery pack is being charged or not. As such, the fully charged condition of the battery pack can be accurately detected even if the tool is operated when the battery pack is almost fully charged.