A cutting insert has a polygonal plate-like insert body defined by a front face, a back face, and a plurality of side faces lying between the front and back faces. A nose portion is defined by one of the corners of the front face. At least one cutting edge extends immediately from the nose portion. The cutting edge is defined by one of a pair of intersections of the front face with an adjacent pair of side faces sandwiching the nose portion therebetween. The front face includes an inclined surface sloping toward the back face in a direction away from the nose portion, causing each of the pair of intersections to incline toward the back face in a direction away from the nose portion. A breaker is formed on the inclined surface along the intersection of the inclined surface with a side face. The breaker has an inner surface for defining a rake surface portion more positively inclined to the corresponding cutting edge than that defined by the other area of the front face. The front face may be of a tetragonal shape with a pair of inclined surfaces that impart a V-shaped contour when viewed along a diagonal line interconnecting the corners of the front face other than those defining a diagonally-opposite pair of nose portions. A breaker on such an inclined surface may have a trough line inclined to a corresponding cutting edge. The insert body includes side faces each sloping along the thickness of the insert body at different angles between various regions of the side face, these regions corresponding to various locations of the front face where there are or are not inclined surfaces and where there are or are not corresponding cutting edges.