A door mechanism of a machining center includes a wide door panel, narrow door panels, a pair of upper and lower slide rails into which the wide door panel and the narrow door panels are fitted, and a plurality of motors that serve as drive units for independently driving the wide door panel and the narrow door panels. The wide door panel and the narrow door panels are arranged in line extending across an opening of the machining center to close the opening of the machining center and are slidable in a direction parallel to a width of the opening to open a portion of the opening having a width approximately twice a width of a one of the narrow door panels.