PURPOSE: To prevent clogging of a matrix with a simple structure by providing both magnetic field generating function and degaussing function to an electromagnet.

CONSTITUTION: An electromagnet 11 generating AC magnetic field or generating DC magnetic field changing periodically to AC is used as an electromagnet for applying magnetic field to a matrix 13. If an AC magnetic field is generated with 4KOe amplitude and 60Hz frequency at an interval corresponding to the distance between the counter magnetic pole 12 of an electromagnet 11, a minutely small part 27 of the matrix 13 is moved in x-direction 26 and is separated apart from a magnetic pole 12, and the amplitude of the AC magnetic field is decreased simultaneously. When the separation is sufficiently large, the amplitude becomes 0KOe, and the residual magnetism of the minutely small part 27 and the particles of magnetic body attracted to the part are degaussed completely. By the process of this degaussing using AC particles of the magnetic body attracted magnetically to the matrix 13 is removed effectively, therefore, the clogging of the matrix 13 is almost completely prevented.