A cutting insert is provided with a plurality of cutting portions formed at the intersection portion of each end surface and a peripheral surface. Each cutting portion includes first and second cutting edges. First corners and second corners which differ in their internal angles are alternately formed in each end surface. An internal angle of the first corner is smaller than an internal angle of the second corner. Each first cutting edge has a portion extending to approach the median plane as a distance from a corresponding first corner increases. A first side surface portion extending on the peripheral surface from each first cutting edge forms an insert internal obtuse angle, and a second side surface portion extending on the peripheral surface from each second cutting edge forms an insert internal acute angle.