Simulation model creating method, mask data creating method and semiconductor device manufacturing method

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.