PURPOSE: To stabilize the dimension of a hole and improve the quality of the hole by measuring the diameter of the hole by an inside diameter measuring device having a double-leaf measuring element after the hole is machined, and automatically adjusting the tool-diameter of a boring-machining tool based on a correction program preliminarily stored in the side of a control device so that the diameter of the hole conforms to the central value of hole tolerance.

CONSTITUTION: After a hole is machined in a work 6, a turntable 5 is rotated, and the work is directed to the side of an inside diameter measuring device 1 for the hole to be automatically measured. The measured results are inputted to a control device 3. The control device calculates the number of strikes of a tool-diameter correcting tool 4 by the difference between the measured values and the central value of tolerance of the hole-diameter based on a preliminarily stored correction program, and indicates the number of strikes to the tool-diameter correcting tool. The tool- diameter correcting tool engages a boring-machining tool 2 to strike the boring- machining tool by the indicated number of strikes, and to automatically correct the diameter of a tool. Thereafter, a next work on the turntable is directed to the boring- machining tool side and positioned. The work is machined by the boring-machining tool, the tool-diameter of which is corrected.