PURPOSE: To permit the detection of whether a phase, in which a current sensor is not provided, is open-phase or not without increasing installation cost remarkably.

CONSTITUTION: An open-phase detector is provided with means 6, 7, deciding the open-phase of two phases provided with current sensors 3, 4, an operating means 9, inputting the value of current of either one phase between two phases and multiplying it with a predetermined scale factor to output it after a predetermined period of time, and a comparing means 10, inputting the output of the operating means 9 and comparing it with the value of current in the present same phase, or an operating means 6, inputting the value of current in either one of the phases with a predetermined time interval and operating the ratio thereof, and a comparing means 10, inputting the output of the operating means 6 and comparing it with a previously stored predetermined scale factors. According to this method, the open-phase of the other one phase can be judged without providing with a current sensor in the other one phase and the burning of a refrigerating device due to the continuation of the operation under the condition of open-phase of the other one phase can be prevented.