To reduce fraction defective by stabilizing machining quality, to suppress machining dispersion due to wear of a cutter blade, and to reduce the replacing frequency of the blade.

In a method for forming a breaking line of half-cut on an instrument panel skin 24 by using the cutter blade by an industrial robot 22, a pressure detecting mechanism 26 is provided on a back surface of the cutter blade 23 to always detect pressure during processing. So as to always fix the pressure, a speed command signal is decided as needed by taking an error due to environment and temperature changes into consideration, and fed back to a robot controller 21, thereby controlling the speed of the industrial robot 22.