To realize a stable temperature control of a circuit element while the life of a fan is prolonged, by a method wherein only in the case where a cooling of the circuit element is needed, the fan is rotated to inhibit a temperature rise of the circuit element.

The temperature of a circuit element mounted on a printed board is detected and at the same time, the amount of a current to flow in the printed board per unit time is detected. When the amount of the current reaches a prescribed amount or more, a fan is rotated to conduct a cooling of the circuit element and after that, when the amount of the current is decreased and the temperature of the circuit element is also reduced, the rotation of the fan is made to stop. Moreover, the amount of the current is a small quantity, but even when the temperature of the circuit element is high, the fan is rotated to conduct the cooling of the circuit element and after that, when the temperature of the circuit element is reduced, the rotation of the fan is made to stop.