To provide a machine tool of which the operation rate can be increased by reducing time for suspending an operation for machining an object to be machined during the object-machining operation using the machine tool. A jig includes at least a first jig (5) disposed in a casing (2) and a second jig (14) by which an object (8) is detachably held. The jigger has attaching/detaching structure (17) for detachably mounting the second jig (14) onto the first jig (5). The second jig (14) is provided with grip parts (32) for gripping the second jig (14) during its movement. The casing (2) is provided with an opening (S) for taking in and out the second jig (14) relatively to the position where the object is to be machined from the outside of the casing (2), and guide passages (18), (22) for guiding generally horizontal movement of the second jigs (14) are provided at least between the above-said position and the opening (S) so that the second jig (14) gripped by the grip parts (32) can be moved along the guide passages (18), (22).