A compact battery pack with high handling ability and restraining degradation of battery cells. The battery pack includes an insertion portion to be inserted into a handle portion of a cordless power tool and an accommodation portion in which all battery cells are accommodatable. A protection board with a protection circuit that protects batteries against overcharge and over-discharge is installed in the insertion portion. A switching element is connected between the battery cells and a drive motor of the power tool. An air passage in communication with the battery pack is formed within the handle and a main housing of the cordless power tool. The switching element is positioned at the air passage.