PURPOSE: To return humidity in a room collected at the time of a cooling cycle in the case of a defrosting cycle into the room by reducing vapor condensation and freezing of the humidity in the room at the time of the cooling cycle.

CONSTITUTION: At the time of a cooling cycle of a cold air type cooler, a fan for supplying air to an outdoor condenser is rotated at a speed steplessly controlled in response to the atmospheric temperature thereby to steadily hold a surface temperature of the cooler at 15°C or higher to cool it. At the time of a defrosting cycle, fans for supplying airs to an indoor cooler and the outdoor condenser are stopped. However, when the surface temperature of the cooler becomes +35°C or higher, the fan for supplying the air to the condenser is rotated at a maximum speed to defrost it. Thus, the inside of a cold storage box, a storage box, etc., can be so cooled to optimum low temperature and high humidity to preserve fruit such as apples, vegetables in the northeastern district where the atmospheric temperature changes from -15°C to +40°C.