A lid of a battery case is supported on a shaft fixed on a battery box. The lid comprises a cover portion and a support arm for attaching the cover portion to the shaft. A hook formed on the cover portion engages with a hook receiver provided on the battery box, so as to retain the lid in a closed position. The hook is disengaged by sliding the lid from the closed position along the axial direction of the shaft, and the lid is subsequently rotated about the shaft to an open position. During rotation of the lid, a stopper abuts a side surface of the support arm to prevent sliding movement of the lid. When the lid is being rotated to be closed, the abutting stopper prevents jamming of the hook on top of the hook receiver.