A power source device may include a casing housing a fuel cell or a battery; a small room provided in the casing, the small room partitioned from a main space in which the fuel cell or the battery is housed; a cooler arranged at a floor of the small room; a first electric component being in contact with a lower surface of the floor so as to be opposed to the cooler; and a second electric component being in contact with an upper surface of the floor so as to be opposed to the cooler, an amount of heat generated by the second electric component being smaller than an amount of heat generated by the first electric component, wherein the casing may include a through hole communicating the small room with the main space.