A magnetic field adjusting device includes a pair of opposing pole plates mounted on respective magnetic field generating sources, forming a magnetic field space a plurality of shimming plugs movably mounted at the periphery of the pole plates, with each shimming plug being mounted in a radially oriented retaining groove, so as to be moveable in the direction of the retaining groove. Additionally, or alternatively, the circumferences of the magnetic field generating sources are arranged with adjusting bars which can move perpendicular to the pole plates.