To improve the battery performance and prolong battery service life by adjusting the temperature of a battery which is mounted on a vehicle.

Plural battery cells 3 are contained in a battery, containing space 9 in a battery case 5 installed on a lower surface of a car body floor of an electric vehicle. A Peltier element for exhibiting heat generation or heat absorbing action by supplied current directions is provided between the battery containing spaces 9 as it is tightly applied to an outer surface of the battery containing space 9. The temperature of the battery cell 3 is detected by a temperature sensor, and the current directions through the Peltier element are changed, depending on the temperature, thereby the battery cell 3 is heated or cooled.