A battery unit mounted in an electric vehicle is provided with a battery case and a plurality of battery modules. The battery case includes a tray member on the lower side and a cover member on the upper side. The tray member is fixed to side members by a beam member extending in a width direction of a vehicle body. Each battery module is composed of a plurality of cells. Among all the cells contained in the battery case, at least those cells on the outer peripheral side which are located near a flank of the vehicle body are located on the tray member so that a lamination direction of cell elements is coincident with the width direction of the vehicle body.