To provide a demagnetizer of construction structures capable of improving workability at a construction site or the like where lifting equipment is not equipped due to the weight reduction of each component apparatus.

A demagnetizer of this invention comprises a direct-current power supply 14 using a step-up transformer 16 and a capacitor 8, a demagnetization coil 1 for remanence, and a current circuit 3 connecting the direct-current power supply with the demagnetization coil 1 in series. The iron core of the step-up transformer 16 comprises a ferrite material to boost the electric power charged to the capacitor 8. The electric power supplied to the step-up transformer 16 is converted to high frequency by a frequency converter 15, so as to improve the charging efficiency to the capacitor 8. A change-over switch 11 is arranged between the step-up transformer 16 of the direct-current power supply 14 and the capacitor 8 to convert the polarity to the demagnetization coil.