To obtain a desired distribution of magnetic fields by laying out magnetic blocks on flat faces facing each other in basic magnetic poles.

This magnetic field device consists of a core 1 shaped like a loop, except for a part of a magnetic field gap G and a coil 2 wound around the core 1. Tips of the core 1 facing the part of the magnetic field gap G are called basic magnetic poles 3. Two basic magnetic poles 3 constituting the gap G of the magnetic field respectively have flat faces facing each other. Then, an area (m) that changes the shape of magnetic poles 3 is placed adjacent to the direction, where the magnetic poles 3 face each other and the shapes of the magnetic poles are fixed by locating one or a plurality of magnetic blocks of a prescribed size in the area (m). These magnetic blocks 3 consist of the core 1 and a magnetic material such as iron, the same as the basic magnetic poles 3, and the blocks may be laid out in an X-direction along flat faces of the basic magnetic poles 3 or may be superposed in a Y-direction that faces the flat faces. Consequently, the shapes of the magnetic poles can be fixed easily and arbitrarily.