In an in-vehicle battery of the present invention, a second battery module (13C) and a third battery module (13R) higher than the second battery module (13C) are disposed below a floor panel of a vehicle. The third battery module (13R) is provided adjacent to a rear side of the second battery module (13C) in a vehicle front-rear direction. The in-vehicle battery also includes a thin heater module (24) provided in each of two end regions, which are located above the third battery module (13R) in a vehicle up-down direction and include no middle region in a vehicle width direction, and configured to heat the third battery module (13R).