To restrict a maximum consumed electrical power to a low value and reduce cost by a method wherein there are provided a thermal insulating door for closing an opening of a thermal insulating box body in such a way that it may be opened or closed, a cooling device for cooling an inner side of the thermal insulating box body and a cool storage medium arranged at an upper part inside the thermal insulating box body and at an inner surface of a thermal insulating door.

Cool storage medium 7 is fixed to an upper part of a storehouse inside 3 and to an inner surface of a thermal insulating door 4. By this arrangement, even after a compressor is stopped in its operation, it is possible to cool the storehouse inside 3 with latent heat of fusion of the medium 7 and to restrict an increasing in temperature. Accordingly, the compressor is stopped in compliance with a time period of 1:00pm to 4:00pm when a consumed electrical power is increased, thereby it becomes possible to reduce the maximum consumed electrical power and realize an efficient low-cost operation. In addition, since the compressor is operated for two hours before the operation of the compressor is stopped with a timer irrespective of the storehouse temperature, it is possible to cool items G in the storehouse and the medium 7 before the compressor is stopped and further a storing performance can be improved more.