The present disclosure provides a method of performing circuit simulation of electrical characteristics of a transistor formed on a semiconductor substrate using calculators, each of which includes a memory. A first calculator receives mask layout data and distance-dependent data indicating a distance from the target transistor. Then, a second calculator calculates an area ratio of a layout pattern of a predetermined mask from the received mask layout data, and calculates a parameter α based on the area ratio and the distance-dependent data. Then, the second calculator B calculates a change ΔP in the electrical characteristics of the transistor based on the parameter α. This configuration provides highly accurate circuit simulation of the electrical characteristics of the transistor, which depend on variations in temperature distribution of the semiconductor substrate during heat treatment due to the mask layout pattern around the transistor.