A simulation system includes an input acceptance unit that accepts a measured dimension of a transfer pattern; a calculation unit including a light intensity calculation unit that calculates a light intensity at each position, and a modified light intensity calculation unit that adds a modified value including the product of the light intensity and a tentative optical reaction coefficient to the light intensity, thereby giving a modified light intensity; and a decision unit that decides the threshold value and optical reaction coefficient by regression calculation such that a difference between the measured dimension and the calculated dimension becomes minimal under the modified light intensity, with a constant being the threshold value of the light intensity at a pair of edges defining the calculated dimension of the transfer pattern in the simulation.