In the present invention, a computer is made to execute the following steps: a first volume distribution of a magnetic material on a shim tray is calculated on the basis of a first magnetic field strength distribution of a magnetic field space (S3) a first composite distribution, representing, for each region of the shim tray, the volume obtained by adding the position thereof and the volume of the magnetic material in the first volume distribution, is acquired (S5) a virtual magnetic field strength distribution created by a magnetic material supposed to be arranged as in the first composite distribution is calculated (S8) a second magnetic field strength distribution, obtained by adding the first magnetic field strength distribution and the virtual magnetic field strength distribution, is calculated (S9) a second volume distribution for the magnetic material in the shim tray is calculated on the basis of the second magnetic field distribution (S3) a second composite distribution, representing, for each region of the shim tray, the volume obtained by adding the position thereof and the volume of the magnetic material in the second volume distribution, is acquired (S5) and the positions and volumes of regions in the first composite distribution and the second composite distribution are displayed (S10).Dans la présente invention, un ordinateur est amené à exécuter les étapes suivantes : une première distribution volumique dun matériau magnétique sur un plateau de cale est calculée sur la base dune première distribution dintensité de champ magnétique dun espace de champ magnétique (S3) une première distribution composite représentant, pour chaque région du plateau de cale, le volume obtenu par addition de la position de celle-ci et du volume du matériau magnétique dans la première distribution volumique, est acquise (S5) une distribution virtuelle dintensité de champ magnétique créée par un matériau magnétique supposé agencé comme dans la première distribution compos
