PURPOSE: To miniaturize a solar battery circuit by preventing the increase of power consumption during the sunshine of a heater for controlling a temperature supplied with power from a non-stabilized power supply using the solar battery circuit as a power supply source.

CONSTITUTION: A solar battery circuit 1b is connected to a heater bus line 13 besides a main bus line 12, and a heater circuit 5 is supplied with power from the heater bus line 13. The number of the solar battery circuits connected to the heater bus line 13 is computed from the resistance value of the heater circuit 5 and minimum necessary power, and set, thus preventing the increase of the power consumption of the heater circuit 5 during sunshine. Accordingly, the solar battery circuits 1a, 1b can be miniaturized.