1. A method for evaluating an optimal irradiation amount of an electron beam for drawing given pattern onto the sample, said method comprising: a step of segmenting the given pattern into a plurality of regions; a step of evaluating, for each of the segmented regions, stored energy that is a substantive irradiation amount of the electron beam; and a step of evaluating the optimal irradiation amount for each of the regions on the basis of a conjugate gradient method using the stored energy evaluated for each of the regions.