To prevent infiltration of water inside of an air-cooling type battery unit having an air-cooling structure and make a cooling duct structure compact with a simple structure in the cooling duct structure of the air-cooling type battery unit.

A case (17) of a cooling fan (15) is projected upward from the top surface of the air-cooling type battery unit (11), a cooling duct (13) is provided so as to extend from the case (17) of the cooling fan (15), and the intermediate portion of the cooling duct (13) is arranged so as to stride over the top part of the case (17) of the cooling fan (15).