In a superconductive magnet device comprising upper and lower vacuum vessels coupled by a coupling conduit, the upper and lower vacuum vessels 9 and the coupling conduit 24 coupling them are made of a high electric-resistance material so that the magnetic flux of gradient magnetic field coils reaches the upper and lower pair of a second heat shields 8 and a coupling conduit 23 coupling the pair of the second heat shields 8. The coupling conduit 23 coupling the upper and lower pair of the second heat shields 8 is provided with a slit (cut) 41 extending in the vertical direction so that no eddy currents occur and thus eddy currents generated asymmetrically in the coupling conduit 23 are reduced. Artifacts caused by asymmetrical eddy currents can by reduced in an MRI apparatus using such a magnet device.