PURPOSE: To ensure the stable air-conditioning operation even in case of an adverse wind by constitution wherein the revolution of outside blower is checked and if it is not exceeding the specified change-over speed of a revolution controlling gear, said revolution of the blower may be accelerated up to a high speed.

CONSTITUTION: During the normal operation, the output contact 11 of tachometer is closed not to actuate the comparator C1. However, in case of an adverse wind, the revolution of outside blower declines and if the revolution falles below the specified value, said output contact 11 is opened to actuate the comparator C1 charging the condenser CT1 through a resistor R7. Consequently the terminal voltage V2+ of the condenser CT1 is boosted to actuate the comparator CT2 after the time set by a timer T1 elapsed. Thus the coil 10a of the speed change- over relay 10 is turned ON through turning the transistor 12 ON to actuate the contact 10b accelerating the outside blower up to high speed revolution. Through these procedures, the stable air-conditioning operation may be ensured even in case of an adverse wind.