To prevent a state of exceeding a critical pressure from being generated by a method wherein a degree of opening of a pressure reducing device is controlled by more than a certain specified value when the pressure exceeds a set pressure and then a pressure within a freezing cycle is reduced.

A temperature at the head part of a compressor 1 is detected by a temperature sensor 7 and then inputted to a control section 71 for controlling a degree of opening of an expansion valve. Inputted information is sent to a deviation comparator 72 for comparing the present temperature with a target temperature, and then an amount of operation of an expansion valve stepping motor is calculated at an expansion valve stepping motor operation calculation part 73. Then, it is commanded to an electric expansion valve stepping motor 41. In turn, a pressure detected by a pressure sensor 6 is judged by a pressure judging section 61 whether or not it is higher or lower than a set pressure. In the case that the detected pressure exceeds the set pressure, information is sent to the control section 71 for controlling a degree of opening of an expansion valve. In this case, a command is directly outputted from the control section 71 for controlling a degree of opening of an expansion valve to the expansion valve stepping motor 41. The expansion valve stepping motor 41 opens the valve only by the predetermined number of steps and decreases a pressure of refrigerant.