To correct the curve of a bar more quickly by multiplying a factor showing the curved form of a beam of a several-order polynomial measured in machining and calculated on the basis of a plurality of element heights by an inverse matrix to calculate a load for correcting the curve, calculating a load for correcting the inclination, and acting it to a machining jig.

A deformation quantity (y) of a beam 12 preliminarily determined when loads F1, F2, F3 are added to the beam 12 is approximated by a quaternary polynomial y=ax4+bx3+cx2+dx+e to determine a matrix K showing the relation of factors (a), (b), (c), (d) of each order of the loads F1, F2, F3. The inverse matrix K' of a partial matrix for determining each factor (a), (b), (c) is calculated, the deformation quantity of the beam 12 is determined by measuring the deformation quantity when the known loads F1, F2, F3 are added to an actual cartridge, and the factors (a), (b), (c) in a quaternary polynomial h=ax4+bx3+cx2+dx+e of element height calculated in lapping are multiplied by K', whereby the loads F1, F2, F3 for deforming the beam 12 in that form can be determined.