Abstract: A machining center includes a first column, a second column, a crossbeam mounted transversely on the first and the second columns, a tool magazine, an upper tool exchange device mounted on the crossbeam, a lower tool exchange device mounted on the tool magazine and a relay tool exchange device. The tool magazine is mounted near the first column and extends transversely. The relay tool exchange device is mounted on the first column and has a guide device, a frame, a tool case and a driving device. The guide device is mounted on the first column and extends longitudinally. The frame is mounted moveably on the guide device. The tool case is mounted the frame and aligned selectively with the upper and the lower tool exchange devices. The driving device is connected to the frame and drives the frame to approach the upper and the lower tool exchange devices.