pieces 3 4 each of an identical outer diameter are disposed at the central portion of the opposing face for each of the yoke plates 1 2 in which a space 10 is defined and superconductive coil 6 is disposed circumferentially only to the lower magnet pole piece 4 in the opposed pair of magnet pole pieces 3 4 since the magnetic field on the side of the magnet pole piece 4 having the superconductive coil 6 disposed circumferentially is increased vertical imbalance is caused in the vertical magnetic field to very much worsen the magnetic field uniformity in view of the above the outer diameter of the lower magnet pole piece 4 having the superconductive coil 6 disposed circumferentially as in the magnetic field generation device shown in fig2 is made greater than the outer diameter of the upper magnet pole piece 3 not having the superconductive coil 6 by which the vertical imbalance in the magnetic field can be improved to make the magnetic field uniformity satisfactory the outer diameter of the lower magnet pole piece 4 having the super conductive coil 6 disposed circumferential may be made greater than the outer diameter of the other magnet pole piece 3 by properly selecting the outer.