Abstract: A defrost thermostat provided with a timer driven cam mechanism for preventing repetition of defrosting operation shortly after the completion thereof which is apt to be caused due to the temperature lowering of the external unit acting as an evaporator during the indoor heating operation. The cam mechanism includes a masking member coaxially mounted on the disc cam to block the cam recess while leaving a gap for permitting an initial defrost operation. During the defrost operation, the cam follower goes out of engagement until it rides over the masking member. Once the defrost operation is completed, it will not be repeated immediately thereafter, thus ensuring stable normal indoor heating operations.