Abstract: A magnet device for MRI system is provided with a main coil assembly including a plurality of ring-like main coils wound around a reel element and a sub-coil assembly including at least one pair of ring-like sub-coils wound around the reel element. All the coils are arranged symmetric with respect to an axial direction of the reel element. The sub-coils are disposed at axially inner predetermined positions apart from the axial outermost main coils. A coil driving element, provided in the device, makes the main coils generate a main magnetic field in the space and makes the sub-coils generate a magnetic field inverse to the main magnetic field to form a high-uniform static magnetic field as a diagnostic space. Magnetomotive forces of the main coils and sub-coils are adjusted so that the diagnostic space having a longer radial axis than its longitudinal axis is formed in the cylindrical space.

Abstract: A magnet apparatus has an active magnetic shield or a cancel coil disposed coaxially with a main coil at an end portion thereof so that the structure of the magnet apparatus is simplified and the both coils are easily assembled together. The outer diameter of the magnet apparatus is rendered small because it is unnecessary to increase the radius of the apparatus. No misalignment between the main coil and the cancel coil occurs due to an assembly error and/or a magnetomotive force generated when the coils are excited, whereby the static magnetic field is prevented from being distributed unevenly and the tomographic image is refrained from being deteriorated.