PURPOSE: To heighten the ability of cooling down by providing an opening degree calculating means whereby the opening of an expansion valve is calculated so that an actual superheat degree becomes at a preset superheat degree, on the basis of the difference between a value detected by a superheat degree-detecting means and a preset superheat degree calculated by a preset superheat degree-operating means.

CONSTITUTION: The difference between a temperature detected by a temperature-detecting device 100 and a target temperature preset with a temperature-presetting device 200, is operated by a temperature difference calculating means 300. When the temperature difference is at a definite value or higher, preset superheat degree for an evaporator is fixed at a large value until the temperature difference becomes below the definite value, and when the temperature difference is below the definite value, the preset superheat degree for the evaporator is decided corresponding to the temperature difference by a preset superheat degree calculating means. After that, on the basis of the difference between a value detected by a superheat degree-detecting means 500 and a preset superheat degree calculated by the preset superheat degree-operating means 400, the degree of opening of an expansion valve 15 is operated so that an actual superheat degree becomes at the preset superheat degree by an opening degree calculating means 600. On the basis of the result of the calculation, the opening of the expansion valve 15 is driven by a driving means 700 and the ability of cooling down is heightened.