PURPOSE:To enable the measurement of the bore daiameter while inserting a boring quill into the main shaft, by projecting a referential member for measuring the diameter of known size at the tip of a boring quill having a tool position regulating mechanism. CONSTITUTION:A main shaft 12 formed with a taper hole 11 at the tip is supported rotatably on a main shaft head 10 of a neumeric control work machine equipped with an automatic tool exchanger. The mai shaft 12 is rotated by a drive motor and stopped at perdetermined angle position by a costant position stopper when exchanging the tool while detected by a proximity switch for every rotation when correcting the tool. The work machine having the bore diameter measuring function is comprised of a machine body, a neumeric controller, a contact detecting circuit, a bore diameter measurement controller, etc. With such structure the bore diameter can be measured while inserting a boring quill 13 into the main shaft 12.