An insert in which a cutting edge is formed only between an upper surface and a side surface. The insert comprises: three or more corners along a perimeter; a main cutting edge formed on an edge between an upper surface and a main side surface, and a sub cutting edge formed on an edge between the upper surface and a sub side surface, wherein the main cutting edge and the sub cutting edge are placed in succession between the corners; and a flat lower surface. The main cutting edge forms an inclination angle such that a height decreases gradually in a direction toward the sub cutting edge. The main side surface is formed by a first end main side surface and a second end main side surface. The sub side surface is formed by a first end sub side surface and a second end sub side surface.