PURPOSE: To perform accurate cutting while making immediate correction by a controller wherein a sensor is provided inside a milling chuck to always monitor even a slight axial shift of a bit during cutting, if occurred.

CONSTITUTION: A sensor 16 is provided inside a milling chuck 47 to detect a shift of a bit 8 in the axial direction A. A signal from the sensor 16 is input to a user micro (not shown) on the side of a machining tool 5 with a minute given dimension being represented by one pulse, whereupon a spindle 6 of the machining tool to which the milling chuck is mounted is moved in the axial direction A for correction. Thus, machining accuracy of the work can be held high.