PURPOSE: To easily eliminate a mounting mistake by mounting a workpiece to be automatically carried in by a jig each mounting a reference block changing its level height and measuring this block by a measuring device discriminating the workpiece for whether or not it agrees with a work program.

CONSTITUTION: A reference block 3 with a different level height for discriminating a workpiece 2 is mounted to each jig 1 fixed to the workpiece 2. This reference block 3 is measured in its level height by vertically moving a contactor 5 of an automatic size measuring device in a machine tool main unit discriminating the workpiece 2. Accordingly, when the workpiece 2 is automatically carried in and fixed, the workpiece 2 is discriminated deciding whether or not it agrees with a work program, and mismounting can be eliminated.