A magnet field generating apparatus for MRI, comprising:a plurality of permanent magnet assemblies disposed continuously around the angular inner circumference of a hexagonal tubular yoke to provide a uniform magnetic field at the center portion of the hollow space inside said permanent magnet assemblies; the permanent magnet assemblies forming a main magnetic field by a pair of oppositely disposed permanent magnet assemblies which are trapezoidal or square in cross-section; other pairs of permanent magnet assemblies having a triangular or square cross-section; each of the pairs of oppositely disposed permanent magnet assemblies has a side surface in contact with a part of the inner peripheral surface of the yoke and forms a plurality of permanent magnet blocks which are magnetized perpendicularly to the side surface in contact with the yoke so that the permanent magnet assemblies are magnetized perpendicularly to the side surface thereof in contact with the yoke; magnetic pole pieces disposed on each surface of the pair of oppositely disposed permanent magnet assemblies; and permanent magnet piece projections having the same magnetizing direction as the plurality of permanent magnet assemblies are respectively disposed on each surface of the paired oppositely disposed permanent magnet assemblies on which no magnetic pole piece is disposed.