A battery pack includes battery cells, a fan unit that blows air for cooling the battery cells, a case housing therein the battery cells and the fan unit, a circulation passage in which the air blown out from the fan unit flows before being sucked into the fan unit and after exchanging heat with the battery cells, a heat exchanging part that is disposed downstream of battery passages through which the air circulating in the circulation passage as an circulating air flows to exchange heat with the battery cells and below bottom ends of the batteries, and a drain passage that leads to outside of the case for drainage, the drain passage including a drain opening formed in the case so as to be located below the heat exchanging part.