A power supply apparatus that supplies electric power to an external electrical device through a cable is provided with a battery pack, and a battery pack case that houses the battery pack. The battery pack case includes a case body having an opening through which the battery pack to be taken in and out, a cover member that liquid-tightly closes the opening of the case body, in which at least one of the case body and the cover member is formed with a ventilation hole so as to penetrate therethrough, and a filter apparatus that does not allow liquid to pass therethrough but allows gas to pass therethrough is provided in the ventilation hole.