A casing for a battery of the present invention is made up of a top-open box and a lid closing the open top of the box. The lid has a pyramidal top portion formed with a plurality of vent holes for allowing a gas to pass therethrough. A polymeric osmotic film allowing a gas to pass therethrough, but intercepting water, is fitted in each of the vent holes. The casing therefore vents hydrogen output from a battery while preventing water from entering it.