A Computer Aided Manufacturing (CAM) system enables high efficiency and high quality. The CAM system of this invention includes a unit that uses a computer aided design (CAD) model that is solid model data of a metal mold to be made and a measured work geometric model that is geometric model data of a work, which is obtained by measuring a work to be machined, to generate a cutting margin model that is difference between the measured work geometric model and the CAD model, and unit that generates numerical control (NC) data based on the generated cutting margin model. According to this invention, one of ordinary skill in the art can easily machine an actual article in conformity with the actual shop floor in the actual factory to make a metal mold.