To provide the lowest temperature detecting device which can prevent such conditions as a heater is stopped notwithstanding a battery is not sufficient ly warmed up or the heater is actuated notwithstanding it is warmed up or energy is wastefully consumed.

A battery 1 is connected to a battery charger 3 and a battery charger 4 through a junction box 2. Switching is performed in the junction box 2 so that the battery 1 and the battery charger 3 are connected to each other at charging time and the battery 1 and the battery charger 4 are connected to each other at discharging time. The battery 1 is covered with a surrounding case 8, and air warmed up by a heater 9 is sent to the battery 1 by an air sending fan 6, and temperature control can be performed. A temperature of the battery 1 and a temperature of the air sent from the fan 6 are always measured respectively by temperature sensors 7a and 7b. Signals always measured by the temperature sensors 7a and 7b are inputted to a controller 5, and the signals are sent to the battery charger 3, the battery charger 4 and the fan 6 by operation of the controller 5, and control can be performed.