To provide a demagnetizing method of an iron core which is capable of sure demagnetization in a short time by using a small capacity power supply, irrespective of a mode of residual magnetic flux of a three-phase static induction electric machine.

A part between a pair of terminals of a delta winding 1 of a three-phase static induction electric machine is short-circuited with a short- circuit wire 8. A power source 2, capable of free adjustment of an applied waveform, is connected with a part between a terminal different from the terminals of short-phase and the short-circuit wire 8. A resistor 3 is fixed to the applying side. A measuring apparatus 4 measuring an induced voltage waveform of each phase of a star winding 7, and a measuring apparatus 5 measuring a current waveform of each part on the delta winding 1 side are installed. A computer 6 is connected with the measuring apparatuses 4, 5. Through a power source 2, a rectangular wave voltage is generates which attenuates alterntaley in positive and negative polarities and applies the voltage to the delta winding 1. Then other phase is short-circuited, and the similar work is performed.