PURPOSE: To secure objective processing precision without necessitating any lapping correction of a master cam and a regular correcting work of the master cam by providing a driving means for slightly moving a tool in an operation means on the basis of the correction value.

CONSTITUTION: An operation means 20 provided with a tool 22 is related to the copying surface 13 of a cam 12 approximately matching to the shape of a part to be processed in a work, and the work and the cam 12 are synchronized with each other and rotated, thereby the operation means 20 is moved along the copying surface 13. The position of the operation means 20 is detected by a detecting means 30 during movement of the operation means 20, thereby the actual value of the shape of the copying surface 13 is obtained from the position of the operation means 20 during movement of the operation means 20. The difference value is found by comparing this actual value with the theoretical value of the shape of the copying surface 13, and the reciprocal of the difference value is found as the correction value. A driving means 24 is driven on the basis of the correction value, and the tool 22 is slightly moved in the operation means 20, moreover the part to be processed in the work is processed.