To avoid increase in consumption of power and deficiency in cooling by changing cycle time of defrosting operation, until a nest defrosting operation starts according to the quantity of frost.

Return time is counted by a return time counting means 17 from the time of starting defrosting operation by driving a heater 11, until a cooler 8 returns to a cooling state, after the temperature of the cooler 8 detected by a second temperature sensor 19 reaches a preset temperature, and the cycle time until the succeeding defrosting operation starts is changed, according to the return time counted by the return time counting means 17.