A cutting insert comprising: an upper surface; a lower surface on an opposite side to the upper surface; and a circumferential side surface connecting the upper surface and the lower surface to each other, wherein a ridgeline at which the upper surface and the circumferential side surface intersect each other includes a plurality of corners (A, B, C, D, E, F, G, H) disposed at equal intervals and a plurality of cutting edges disposed one by one between the corners adjacent to each other, the cutting insert is a regular polygon having six or more sides, the cutting edges include a first cutting edge that is linearly formed and a second cutting edge that is linearly formed, the second cutting edge has a length equal to or less than half a length of the first cutting edge and is positioned closer to a center side of the cutting insert than an extension line of the first cutting edge is, the circumferential side surface includes a first side surface facing the first cutting edge and a second side surface facing the second cutting edge, and a clearance angle (α) of the first side surface is smaller than that (β) of the second side surface, wherein the second side surface extends to a side opposite to the first cutting edge with the first side surface interposed therebetween and the first side surface is formed to be narrower with increasing separation from the corner adjacent thereto, characterised in that the circumferential side surface is ground, the second side surface extends to a region below the first side surface, the circumferential side surface further includes a third side surface facing, from the opposite side to the cutting edge, the second side surface, and the third side surface is formed as a curved surface recessed toward the center side of the cutting insert. a ridgeline at which the upper surface and the circumferential side surface intersect each other includes a plurality of corners (A, B, C, D, E, F, G, H) disposed at equal intervals and a plurality of cutting edges disposed one by one between the corners adjacent to each other, the cutting insert is a regular polygon having six or more sides, the cutting edges include a first cutting edge that is linearly formed and a second cutting edge that is linearly formed, the second cutting edge has a length equal to or less than half a length of the first cutting edge and is positioned closer to a center side of the cutting insert than an extension line of the first cutting edge is, the circumferential side surface includes a first side surface facing the first cutting edge and a second side surface facing the second cutting edge, and a clearance angle (α) of the first side surface is smaller than that (β) of the second side surface, wherein the second side surface extends to a side opposite to the first cutting edge with the first side surface interposed therebetween and the first side surface is formed to be narrower with increasing separation from the corner adjacent thereto, characterised in that the circumferential side surface is ground, the second side surface extends to a region below the first side surface, the circumferential side surface further includes a third side surface facing, from the opposite side to the cutting edge, the second side surface, and the third side surface is formed as a curved surface recessed toward the center side of the cutting insert.