If an abnormality such as breakage caused by stress due to aging occurs on the rotor of an induction motor, unbalanced current flows in the rotor and load current of the motor pulsates as a result. When unbalanced current flows, sideband waves ±2sf are generated with respect to the fundamental wave f on the current spectrum as shown in the following figure. This pulsation current ratio is almost proportional to the extent of rotor abnormality. By measuring pulsation current with a clamp type ammeter and analyzing frequency spectrum variation of the pulsation current with an FFT analyzer, breakage of a bar or a rotor can be diagnosed.