Disclosed herein is an automatic pallet exchanger of a machine tool, which can exactly decide a position of a pallet exchanging operation, and which can improve stability in operation. The automatic pallet exchanger includes an APC driving shaft (30) rotatably and elevatably standing on an APC body (20); an APC arm (40) supported on the APC shaft (30) for exchanging pallets of a processing area and a waiting area; rotating means for rotating the APC driving shaft (30); a cylinder (80) inserted on the APC driving shaft (30) and fixed on the APC body; a piston mounted on the APC driving shaft (30) for elevating the APC driving shaft (30) according to hydraulic pressure direction within the cylinder (80); a stopper (100) located within a fixed radius from the APC driving shaft (30) and fixed on the APC arm (40); and a pair of stop bolts (110A, 110B) fixed on the APC body (20) to stop the rotation of the APC arm (40) by contacting with the stopper (100) whenever the APC arm (40) is rotated at an angle of 180°.