an electrically charged beam irradiating mechanism;

a coefficient calculating section which, based on an approximating function for approximating a relationship between a global pattern coating rate in a range wider than that of a proximity effect and a backward scattering coefficient and based on a global coating rate of a pattern, calculates the backward scattering coefficient of the pattern; and

an irradiation quantity calculating section which, based on the backward scattering coefficient of the pattern, calculates a beam irradiation quantity used for drawing the pattern using the electrically charged beams;

wherein the coefficient calculating section calculates the backward scattering coefficient, which is represented by ηM, based on a formula showing a relationship between the global pattern coating rate, which is represented by Z, and the backward scattering coefficient ηM, i.e., ηM=C1×Z+C2×log(Z+1)+C3, where C1, C2, and C3 denote constants, respectively; and

wherein the coefficient calculating section include a unit which obtains the approximating function formula using a check mask formed on a check mask substrate, wherein the check mask includes a plurality of check patterns having similar shapes with each other, the check patterns includes a first pattern having a plurality of proximity effect areas with different local coating rates, the first pattern is formed in a first area of 0% global coating rate, and the remaining patterns include a plurality of fogging effect areas with different global coating rates selected from a value not less than 0% and not larger than 100%.