A battery pack includes batteries, a fan unit, a case housing the batteries and the fan unit, an air circulation passage formed in the case, an air introduction passage that makes communication between a suction part of the fan unit and outside of the case, and an air discharge passage that makes communication between the outside and an inside of the case. The air circulating passage includes a blow-off side passage that makes communication between a blowoff part of the fan unit and battery passages through which the circulating air flows to exchange heat with the batteries, and a suction side passage that makes communication between the battery passages and the suction part of the fan unit. At least one of the blow-off side passage and the suction side passage is an in-duct passage formed inside a duct disposed in the case.