A battery pack mounted under a floor of a vehicle body and containing a plurality of battery modules (17, 18), the battery pack comprising: a tray having a substantially rectangular shape in a plan view and containing the battery modules (17, 18), and a cover which is liquid-tightly attached to an upper face of the tray, a center of the tray in the vehicle width direction being substantially aligned with a center of the vehicle body in a vehicle width direction, wherein the battery modules (17, 18) inside the tray are arranged adjacent to each other in a plurality of rows in a vehicle anteroposterior direction, at both sides of the center of the tray in the vehicle width direction, in such a manner that a longitudinal direction of each battery module (17, 18) corresponds to a vehicle width direction, each battery module is provided with electrode terminals including a positive electrode terminal and a negative electrode terminal provided at a position lower than an upper face of the battery module, in both end parts thereof in the vehicle width direction, the positive electrode terminal and the negative electrode terminal of the adjacent battery modules being adjacent to each other, bus bars (43, 45) for electrically connecting in series the positive electrode terminal and the negative electrode terminal of the adjacent battery modules include outer bus bars for interconnecting the electrode terminals in an outer part in the vehicle width direction, and inner bus bars for interconnecting the electrode terminals in a center part in the vehicle width direction, characterized in that the whole of the outer bus bars are provided so as not to protrude from upper faces of the battery modules (17a, 17b) thereby to interconnect the electrode terminals of the adjacent battery modules (17a, 17b), and the inner bus bars are provided so as to pass above the upper faces of the battery modules thereby to interconnect the electrode terminals of the adjacent battery modules. a tray having a substantially rectangular shape in a plan view and containing the battery modules (17, 18), and a cover which is liquid-tightly attached to an upper face of the tray, a center of the tray in the vehicle width direction being substantially aligned with a center of the vehicle body in a vehicle width direction, wherein the battery modules (17, 18) inside the tray are arranged adjacent to each other in a plurality of rows in a vehicle anteroposterior direction, at both sides of the center of the tray in the vehicle width direction, in such a manner that a longitudinal direction of each battery module (17, 18) corresponds to a vehicle width direction, each battery module is provided with electrode terminals including a positive electrode terminal and a negative electrode terminal provided at a position lower than an upper face of the battery module, in both end parts thereof in the vehicle width direction, the positive electrode terminal and the negative electrode terminal of the adjacent battery modules being adjacent to each other, bus bars (43, 45) for electrically connecting in series the positive electrode terminal and the negative electrode terminal of the adjacent battery modules include outer bus bars for interconnecting the electrode terminals in an outer part in the vehicle width direction, and inner bus bars for interconnecting the electrode terminals in a center part in the vehicle width direction,