A heater module is configured to be arranged on a side surface of a battery module and includes: a heat equalizing plate; a heater, a heater cover; and a metal stay in this order from the side surface of the battery module. The heater is fixed on the heat equalizing plate, the heater cover includes a heat equalizing plate holding portion and a stay holding portion, the metal stay includes: fastening portions provided on both end portions thereof in a stacking direction of cells and configured to be fastened to the side surface of the battery module; and a bent portion configured to cover a lower surface of the heater cover, and the bent portion of the metal stay is arranged at a location overlapping with the heat equalizing plate holding portion of the heater cover in the stacking direction.