Pattern formation simulations are performed based on design layout data subjected to OPC processing with a plurality of process parameters set in process conditions. A worst condition of the process conditions is calculated based on risk points extracted from simulation results. The design layout data or the OPC processing is changed such that when a pattern is formed under the worst condition based on the changed design layout data or the changed OPC processing a number of the risk points or a risk degree of the risk points of the pattern is smaller than the simulation result.