A lithium-ion battery device comprises a housing, a battery cell stored inside the housing, and an adjacent member provided adjacently to and on a rearward side, in a vehicle longitudinal direction, of the battery cell. The battery cell comprises a cell case, an electrode body stored inside the cell case, and a pair of terminals. The cell case includes a first main face and a second main face which face to each other in the vehicle longitudinal direction. At least two gap-forming portions which respectively have a smaller area than the second main face and have a specified thickness of the vehicle longitudinal direction are arranged between the second main face and the adjacent member. The two gap-forming portions are provided to be spaced apart from each other in an inter-terminal direction of the battery cell.