a simulation model creating method computes , for measurement results of a line width of a resist pattern formed with varied an exposure amount and focus value , a permissible fluctuation range of the pattern line width from a distribution of the exposure amount and a distribution of the focus value ; computes difference values between the measurement results and corresponding approximation values on a fitting function which has the exposure amount and focus value as parameters ; compares the difference values with the permissible fluctuation range ; deletes any measurement values for which the difference value is larger than the permissible fluctuation range , and recomputes the fitting function accordingly ; and deletes measurement values outside a permissible fluctuation range of a pattern line width of the mask , and creates a simulation model .