A magnetic field generating apparatus for a magnetic resonance imaging (MRI) system adopting a C-shaped open magnet support structure includes a pair of polygonal permanent magnets disposed parallel to each other in the horizontal plane, with a predetermined imaging area therebetween; a pair of polygonal magnetic pole plates, stacked to face each other on the inner sides of the pair of permanent magnets, respectively; a pair of yokes to which the permanent magnets are fixed; at least one column for connecting the two yokes at one ends thereof with the imaging area being interposed between the yokes, wherein the at least one column together with the pair of yokes forms a closed path of the magnetic field; shims disposed at the vertexes of the polygonal magnetic pole plates, the shims having a polygonal shape, wherein the shims of one of the polygonal magnetic pole plates face those of the other polygonal magnetic pole plate; peripheral permanent magnets disposed along the edges of the polygonal magnetic pole plates, wherein the peripheral permanent magnets of one of the polygonal magnetic pole plates face those of the other polygonal magnetic pole plate; and booster shims arranged in the spaces between adjacent peripheral permanent magnets disposed at the vertexes of the polygonal magnetic pole plates.