To detect a divergent tendency of power fluctuations properly for the power fluctuations including an complex amplitude waveform which occurs actually by determining that the power fluctuation tends to diverge when a difference between the present power fluctuation amplitude value and the second power fluctuation value computed within a prescribed period after the present exceeds a prescribed value.

A power fluctuation amplitude value computing part 18 obtains a difference between a maximum value outputted from a maximum value detecting function 17P and a minimum value outputted from a minimum value detecting function 17N, and regards the difference as a power fluctuation amplitude value to be outputted to a power fluctuation detecting function 19. The power fluctuation detecting function 19 obtains a difference between the power fluctuation amplitude value outputted from the power fluctuation amplitude value computing part 18 and a power fluctuation amplitude value outputted before a prescribed time in the same way, and determines that the power fluctuation tends to diverge when the difference exceeds a prescribed value. It is thus possible to detect that the power fluctuation tends to diverge positively even if the power fluctuation has the complex divergent tendency.