energized which in turn operates magnetic clutch 8 when temperature tth of the air blown by evaporator 5 is lower than predetermined temperature t 2 the output voltage level of comparator 12 is low because temperature voltage vb is lower than reference voltage va transistor tr 1 then is in a nonconductive state and relay ry 1 and magnetic clutch 8 are not energized feed back resistor r 12 is also connected to comparator 12 to provided a hysteresis effect at the output of comparator 12 thus when temperature voltage vb decreases from a level higher than reference voltage va corresponding to a decrease in temperature tth the output of comparator 12 changes from a high level to a low level when temperature voltage vb equal reference voltage va however when temperature voltage vb increases from a level equal to reference voltage va corresponding to an increase in temperature tth the output of comparator 12 does not change from a low level to a high level until temperature voltage vb is higher than reference voltage va by an amount determined by the resistance of resistor r 12 thus predetermined temperature t 1 may be established compressor control circuit 10 also includes circuits for reducing and increasing the cooling capacity of compressor 1 the circuit for reducing the cooling capacity of compressor 1 includes comparator 13 which is coupled to the output.