An insert boring tool includes a tool body and a cutting insert releasably mounted thereon. The insert includes a plate defined by front and rear surfaces and a plurality of side faces and having a plurality of corners where adjacent side faces intersect one another. At least one of the corners is removed to provide a corner face. A first cutting edge is defined by the front surface and one of the side faces adjacent to the corner face, and a straight second cutting edge is defined by the front surface and the corner side face. The first and second cutting edges intersect each other at an obtuse angle, and are indexed in an end cutting position. The front surface has a marginal portion extending along the second cutting edge and inclined in such a manner that a thickness of the plate between the marginal portion and the rear surface is progressively reduced toward the second cutting edge. The axis of the body passes through the second cutting edge so that the first cutting edge and at least a part of the second cutting edge are indexed in an end cutting position.