An apparatus, which controls the temperature of a secondary battery formed by combination of a plurality of single cells or a plurality of battery modules each made by series connection of multiple single cells, prevents variations in the temperature or voltage of the single cells or the battery modules, which could otherwise be caused when the secondary battery is heated. A temperature control section controls the quantity of heat by means of which a heater heats a secondary battery formed by combination of a plurality of battery modules made by series connection of multiple single cells. The temperature control section detects a rate of temporal changes in an open circuit voltage of the secondary battery. When a detected rate of temporal changes in open circuit voltage has exceeded a predetermined threshold voltage value, the heater is controlled to thus diminish the quantity of heat used for heating the secondary battery.