first to fig1 a magnetic field generator for mri 10 as an embodiment of this invention comprises an upper magnet unit 11 and lower magnet unit 12 each of the magnet units 11 and 12 includes but is not limited to a yoke plate 14 a permanent magnet 16 and a pole piece 18 each of the yoke plate 14 has a surface opposed to the other yoke plate and this surface is provided with a permanent magnet 16 on which a pole piece 18 is provided each of the permanent magnets 16 includes a plurality of magnet blocks 20 each of the magnet blocks 20 of the magnet unit 12 is fitted with adjacent ones with the same magnetic pole facing upward on the other hand each of the magnet blocks 20 of the magnet unit 11 is fitted with adjacent ones with the other magnetic pole facing downward in other words the permanent magnet 16 of the magnet unit 12 and the permanent magnet 16 of the magnet unit 11 are faced to each other so that different magnetic poles are opposed to each other the magnet blocks 20 may be a magnet made from a ternary system compound nd u2014 fe u2014 b composed.