A cutting tool for machining large workpieces having relatively a large cutting amount and a cutting insert configured to be used with the cutting tool. The cutting tool includes a cutting insert and a wedge for clamping the cutting insert. The cutting insert has a polygonal top surface, which comprises a recessed portion having a plurality of inclined abutment surfaces which are inclined toward the center of the top surface. The outline of said recessed portion is twisted at a predetermined angle with respect to the outline of the top surface. The wedge includes a projecting portion that contacts the inclined abutment surface and pushes the cutting insert so that two side surfaces of the cutting insert can be supported by a radial inner side surface and a lateral supporting surface of an insert seat.