PURPOSE: To eliminate the need of providing a potentiometer and to improve the durability of the air conditioner by a method wherein a difference between a room temperature and a set temperature is calculated and then compared with a set temperature difference range so as to control through an electric-pneumatic converter the establishment of a communication between a pneumatic mechanism and a pneumatic source.

CONSTITUTION: The difference between the temperature detected by a temperature detector 19 and the set temperature in a set temperature device 18 is calculated in a control circuit 20 and then compared with the preset temperature difference range. If the comparison result is below the first set point, the pneumatic mechanism 7 is interrupted from the pneumatic source 8 and the atmosphere. If the comparison result is above the first set point and below the second set point which is higher than the first set point, the pneumatic mechanism 7 is communicated with the pneumatic source 8 through a throttle. If the comparison result is higher than the second set point, the pneumatic mechanism 7 is directly communicated with the pneumatic source 8. Such a construction eliminates the provision of the potentiometer, which is very advantageous in that the air conditioner is likely to fall in trouble more rarely and the durability thereof is much increased.