PROBLEM TO BE SOLVED: To machine-work a workpiece 5 having a resin containing membrane 18 on the surface of a metal substrate, without damaging the resin containing membrane 18.SOLUTION: A masking paper 20 is so adhered downward from above a resin containing membrane 18 to the surface of said metal substrate as to cover the surface of said metal substrate 18 at least partially, and said metal substrate is machined together with the masking paper 20.SELECTED DRAWING: Figure 2