A cutting insert has top and bottom surfaces opposing each other and a side surface connecting them. A part of an intersecting ridge between the top and side surfaces functions as a cutting edge. On the side surface, a side surface portion adjacent to the intersecting ridge portion opposing the cutting edge intersects the top surface at an acute angle. At least on a part of a bottom surface, an inclined surface gradually approaching the top surface with increasing distance from a side surface portion on a side opposite to a cutting edge is formed. This allows a part of the cutting resistance to act on the side surface portion. As a result, friction resistance between the side surface portion and the side surface of an insert seat becomes larger, thereby suppressing a shifting of the cutting insert when cutting.