TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Structure: In the structure, the battery case includes a rearward portion, a frontward portion and a middle portion, the rearward, frontward and middle portions having different heights, with the height of the rearward portion being highest, the height of the frontward portion being next highest and the height of the middle portion being lowest; and the cooling jacket is disposed on the frontward portion of the battery case. The battery includes first, second and third battery groups, with the first battery group being arranged in a frontward position relative to the second and third battery groups, the second battery group being arranged between the first and third battery groups, and the third battery group being arranged in a rearward position relative to the first and second battery groups, and the battery case includes a first cover portion covering the first battery group with the cooling jacket disposed, a second cover portion covering the second battery group and a third cover portion covering the third battery group, the upper surface of the battery case being in close proximity to upper ends of the first, second and third battery groups and with the first, second and third cover portions having different heights so that the third cover portion is higher than the first cover portion, which is higher than second cover portion. The floor panel and the upper surface of the battery case are vertical spaced apart to define an air flow passage that allows an air stream produced by motion of the vehicle to pass, with the cooling jacket being exposed to the air flow passage. The floor panel includes an upwardly protruding tunnel section that extends longitudinally with respect to the vehicle body so that at least a portion of the air flow passage is formed by the tunnel section. The upper surface of the battery case includes a downwardly recessed trough shaped section that extends longitudinally with respect to the vehicle body so that at least a portion of the air flow passage is formed by the trough shaped section. The upper surface of the battery case at least partially includes a thermally conductive material, with the cooling jacket being arranged on a widthwise-middle portion of the upper surface of the battery case. The upper surface of the battery case protrudes upward at the widthwise middle portion to correspond to a highest position of the upper surface of the battery case. The battery unit and the cooling jacket are provided as a single integrated unit that is detachably and reinstallably attached to vehicle body frame member.