A battery unit is provided which includes a battery and a storage case in which the battery is mounted. The storage case is made up of a base, a cover, and an intermediate case. The cover is jointed to the base with an interspace therebetween. The intermediate case is disposed between the cover and the base and has a peripheral wall which occupies the interspace. The intermediate case is lower in stiffness than the cover and the base, thereby realizing a lightweight structure of the storage case without sacrificing the mechanical strength thereof.