A computer executes: calculating a first volume distribution (v.d.) of magnetic materials on a shim tray, based on a first magnetic field strength distribution (m.f.s.d.) in a magnetic field space (S3) acquiring a first composite distribution (c.d.) representing a volume by addition of volumes of magnetic materials for each region of the shim tray, and positions of the regions (S5) calculating a virtual m.f.s.d. created by magnetic materials supposed to be arranged as in the first c.d. (S8) calculating a second m.f.s.d. by addition of the first m.f.s.d. and the virtual m.f.s.d. (S9) calculating a second v.d. of magnetic materials on the shim tray, based on the second m.f.s.d. (S3) acquiring a second c.d. representing a volume by addition of volumes of magnetic materials for each region, and positions of the regions (S5) and displaying the positions of regions and the volumes in the first c.d. and second c.d. (S10).
