This invention provides a casing for storing MEA, which has satisfactory corrosion resistance to formic acid produced in an electrode reaction of MEA. There is also provided a casing formed of a material having the lowest possible specific gravity that can apply a suitable pushing pressure to MEA and a current collector without increasing the thickness dimension and is suitable for a power supply mounted, for example, in small portable electronic equipment. The casing is a casing for use in a fuel battery in which a hydrogen electrode, an oxygen electrode, and a film-electrode joint product formed of a proton conductive film held between the hydrogen electrode and the oxygen electrode are housed within the casing and, in removing water produced in an oxygen electrode reaction, the above water comes into contact with the casing. The casing for a fuel battery comprises a base material (2a) formed of a magnesium alloy and a film (2b) provided on the base material (2a). The film (2b) comprises a conversion treated film (2ba), an undercoating layer (2bb), and a top coating layer (2bd) provided in that order from the base material side.