PURPOSE: To make it possible to excellently demagnetize even when the objective substance is volumious or it is intensely magnetized by a method wherein an alternative current is applied in such a manner that the current value will successively by reduced rectilinearly from the prescribed value by applying a DC voltage to a demagnetization coil alternately.

CONSTITUTION: DC application voltage V is applied in such a manner that the application period will be decreased gradually while a suspension period t0 is being interposed, so that the peak value of demagnetizing current I may decrease linearly. A working surface 1b is provided on the upper surface of a case main body 1a in a core type demagnetizer, and a U-shaped core 2 is provided on the lower part of the working surface 1b is such a manner that the opened part comes in contact with the lower surface of the working surface 1b. A coil 3 is wound on the above-mentioned core 2, and power is fed from a control device 4 through the intermediary of a cord 5a. Power is fed to the control device 4 from a commercial power source through a cord 5b.