A processing simulation method and apparatus is provided, which can appropriately detect interference between a tool processing area and a shape model of a material without being affected by the accuracy of expression of a tool movement path and the shape model. A tool shape model for processing a material, that includes a strict tool shape, and a tool shape model for checking interference, that is included in the strict tool shape, are generated by tool model setting unit according to an error range set in consideration of the tool movement path and the expression accuracy of the shape model, and the processed material shape model is generated by generating a tool processing area shape model from the tool movement path during processing feed and the tool shape model for processing the material and removing the tool processing area shape model from the material shape model. The tool processing area shape model is generated from a tool movement path during fast feed and the tool shape model for detecting the interference, and the interference between the tool processing area shape model and the material shape model is detected.