Magnetic-field generator comprising a pole-piece unit

A magnetic field generator comprises a pair of pole-piece units. The pair of pole-piece units respectively include plate yokes (12a,12b). Each of the plate yokes includes a surface facing the other's and provided with a permanent magnet group and a pole piece. The pole piece includes an annular projection (32) having a plurality of annular-projection pieces (34) and a permanent magnet (40) for reduction of magnetic flux leakage installed on an outside surface of each annular-projection piece. When assembling the pole piece, first, the permanent magnet for reduction of magnetic flux leakage is fixed on the outside surface of each annular-projection piece (34). At this time, the permanent magnet is slid on the flat outside surface of the annular-projection piece, to a desired position on the annular-projection piece, and then fixed. Each of the annular-projection pieces mounted with the permanent magnet is fixed on a base plate. At this time, the annular-projection piece is lowered on a guide stake attached on the base plate, allowing the stake into a guide hole formed in the annular-projection piece, thereby guiding the annular-projection piece onto the base plate.