PURPOSE: To prevent the clogging of a matrix, by reciprocally moving the matrix by belts for pressing both side surfaces of the matrix within a magnetic field and spraying pressure water to both side surfaces and the upper surface of the matrix in the outside of the magnetic field to wash the same.

CONSTITUTION: Both side surfaces of a matrix 13 are pressed by belts 29 through shafts 28 in the magnetic space between the opposed magnets of a magnetic circuit 11 and the belts 29 are reciprocally moved in a state separated from the matrix 13 in the outside of the magnetic field. Because of this, the matrix 13 reciprocally moves without sliding with the belts. Raw water 17 is supplied to the matrix 13 in the magnetic field from a liquid supply port 16 and magnetic particles 18 are attracted and adsorbed with the matrix 13 while treated water is flowed into a trough 19. Magnetic substances collected by the matrix 13 are washed off by pressure waters 26, 27, 21 sprayed to both side surfaces and the upper surface of the matrix 13 from washing mechanisms at three places in the outside of the magnetic field.