PURPOSE: To improve the machined quality of a long workpiece by providing plural sets of positioning devices in the feed direction of the long workpiece, and engaging each positioning device with a positioning jig mounted at the tip of the workpiece to position the workpiece for machining it per machining division.

CONSTITUTION: In a machine tool to machine a long workpiece 6 such as the chassis frame of a large type vehicle by a machining center 2 having spindles 8 arranged in three directions on whrcle a drill 9 or the like, is detachably mounted a positioning jig 3 is mounted at the tip of the workpiece 6, and it is also attached onto an endless chain conveyor 12. In addition, plural sets of positioning devices 4 provided in the feed direction of the workpiece are arranged at each interval of a distance shorter than the distance in the feed direction of the processing range of the machining center 2. The positioning section 3b of the positioning jig 3 is engaged with the positioning device 4 to position the workpiece 6 in its feed direction for machining it.