A magnetic field generator comprises a pair of plate yokes connected by a column yoke. The pair of plate yokes has a pair of opposed surfaces each provided with a permanent magnet including a plurality of neodymium magnets. In a method in which the neodymium magnets are demagnetized and adhesive is ground, the magnetic field generator is heated to 200.degree. C..about.350.degree. C. After the neodymium magnets are demagnetized, the adhesive is removed and the neodymium magnets are removed and collected from the magnetic field generator. In a method in which the adhesive is carbonized, the magnetic field generator is heated to 350.degree. C..about.1000.degree. C. After carbonizing the adhesive, the neodymium magnets are removed and collected. Surfaces of the collected neodymium magnets are polished and the neodymium magnets are reused. Further, the collected neodymium magnets are re-aged and reused.