PURPOSE: To make a secure open phase decision by sampling the three-phase current of a three-phase power source at intervals of 30° in synchronization with the three-phase power source, finding an in-phase current and an opposite- phase current from the sampling data, and finding the unbalance rate from the currents and deciding an open phase state.

CONSTITUTION: A 1st means 1 samples the three-phase current with an R, an S, and a T phase and stores the sampled data in a RAM. A 2nd means 2 computes crest values of the three R, S, and T phases from the data obtained by the means 1 and find the maximum crest value among the respective crest values. A 3rd means 3 adds data of two phases of 120 and 240° of an actual axial component based upon data of the phase of the maximum crest value as a start point to perform square-law arithmetic. Further, a 4th means 4 adds subsequent data of a virtual axial component with a 90° phase different to data of two phases of 210 and 330° and squares the sum value. A 5th means 5 divides an opposite-phase current by an in-phase current so as to obtain an unbalance rate α, which is compared 6 with a previously set unbalance rate α0, so that an open phase process 7 is performed based on the comparison result.