The power source contains a plurality of batteries stacked in more than three tiers within a battery case divided into a first sub holder case and second sub holder case, and the first sub holder case contains the batteries in a larger number of tiers than the second sub holder case. An intermediary duct is provided between the two sub holder cases, and a first outer duct and a second outer duct are respectively provided outside each of the two sub holder cases. The batteries in the holder case are cooled by the cooling air which flows from the intermediary duct through the holder case into the outer duct for exhaustion. Further, the second outer duct is designed to experience a larger pressure loss than the first outer duct.