A magnetic field generating device for use in MRI having a pair of pole pieces opposed at a space to each other and generating a magnetic field in the space, wherein the pole piece has a laminate structure in which a soft ferrite and a magnetic material base are disposed from the side of the space, and a layer of small magnetic permeability and high electric resistance is interposed between the soft ferrite and the magnetic material base. The soft ferrite may be lamination of the soft ferrite and silicon steel sheets. High speed image pick-up at high sensitivity and sharpness is enabled by reducing the eddy current and the remanent magnetism generated in the pole piece by the gradient magnetic field coils.