PURPOSE: To improve the flatness of a cut surface so as to finish the cut surface with each member ground by means of a grinding wheel having broader width, by cutting a material with a narrow-width grinding wheel having given width, in cutting a prismatic material from platelike magnetic core material, and so on.

CONSTITUTION: A C-core bar block 1 is fixed to a dummy block 3, and the dummy block 3 is fixed to the table on a sliding machine. Grooves 3a, 3a... are provided in the cutting-corresponding positions for the dummy block 3 so as not to be contacted with diamond cutting wheels 4 and 5. A narrow-width grinding wheel, having e.g. 0.7mm width, and diamond cutting wheels 4 and 5, having a broad width of e.g. 1.0mm, are coaxially arranged on a grinding wheel assembly 21 with a width-direction center distance P set to a given one, e.g. 7.5mm. At first, the C-core bar block 1 is cut in given width with the diamond cutting wheel 4, and then a cut surface is cutting-finished with the cutting wheel 5.