PURPOSE: To prevent the break of a machining tool in the rushing stage thereof without any increase in man-hour for preparing machining data by adding a preliminary bore machining data generation part and controlling a contour processing machine on the basis of the machining data and a rush point preliminary bore machining data.

CONSTITUTION: Upon the start of a contour machining, the CPU 31 of a preliminary bore machining data generation part 3 receives machining data from a data input part 1, reads the data and causes RAM 33 to store the data of a machining tool diameter. Then, upon reading machining start point data about a machining shape from the input part 1, the CPU 31 generates rush point preliminary bore machining data having the same diameter value as the machining tool and delivers this data to a contour machining control part 2, using the same coordinate value as a machining start point position, when the machining tool diameter stored in RAM 33 is smaller than a boundary reference value saved in ROM 32. Subsequent thereto, the machining start point data as machining data is delivered to the contour machining control part 2 without any modification.