PURPOSE: To use an integrated value at this time as an initial value for an operation when the initial value of data during a definite time at the last time is zero, to prevent errors from being accumulated and to process the title method at high speed when data sampled at a definite phase angle is operated as an integrated value by oldest data and update data at definite time intervals.

CONSTITUTION: An electric current value at every phase angle of 30° is sampled and digitized. At Step(ST) 31, update data X and oldest data Y are read. At ST 32, an integrated value S for overload judgment use at the last time is taken out, and S=S-X+Y is operated and stored. At ST 33, an integrated value S' for renewal use at the previous time is taken out, S'=S'+Y is operated and the integrated value for renewal use is stored. When one minute elapses at ST 34 and it is judged that no overload exists, S+S' is set at ST 36, S'=0 is set at ST 37 and used as an initial value for a next integration period. When it is judged that an overload exists at ST 38 within one minute at ST 34, an overload signal is sent out. Thereby, it is possible to prevent errors from being accumulated, and the processing of the title method is performed at high speed and the reliability of the title method is enhanced.