A positive-type cutting insert mountable on a tool main body rotating around a rotation axis, and having a polygonal plate shape which is rotationally symmetric with respect to a center line extending in a thickness direction, the cutting insert comprising: a rake face configuring one of a pair of polygonal surfaces; a seating surface configuring the other one of the pair of polygonal surfaces; and a side surface connecting the rake face and the seating surface to each other, wherein an intersection ridgeline between the rake face and the side surface has a cutting edge portion including a main cutting edge located in a side portion of the rake face, a subsidiary cutting edge connected to the main cutting edge, and a corner cutting edge connected to the subsidiary cutting edge and located in a corner portion of the rake face, wherein the side surface includes a flank face and a connection surface which are adjacent to each other in the thickness direction via a boundary line, wherein the flank face is located closer to a rake face side than the boundary line, wherein the connection surface is located closer to a seating surface side than the boundary line, and is located outside an extension line of the flank face in a cross section in a normal direction of a ridgeline of the main cutting edge, and wherein in a cross section in a normal direction of each ridgeline of the main cutting edge, the subsidiary cutting edge, and the corner cutting edge, a passage point of the cutting edge portion is set as a point P, a perpendicular line extending from the point P toward an extension line of the seating surface is set as a line segment H, an intersection point between the line segment H and the extension line of the seating surface is set as a point R, an intersection point between an extension line along the flank face and the seating surface is set as a point Q, a passage point of an intersection ridgeline between the connection surface and the seating surface is set as a point Q', a passage point of the boundary line is set as a point M, a perpendicular line extending from the point M toward the line segment H is set as a line segment N, an intersection point between the line segment N and the line segment H is set as a point M', characterized in that the point Q' is located closer to a point Q side than a point S located at a distance of 1/3 of a line segment QR from the point R, the point M' is located closer to a point R side than a point T located at a distance of 2/3 of the line segment H from the point R, the seating surface and a portion of the flank face which is connected to the main cutting edge function as restriction portions in contact with the tool main body. a rake face configuring one of a pair of polygonal surfaces; a seating surface configuring the other one of the pair of polygonal surfaces; and a side surface connecting the rake face and the seating surface to each other, wherein an intersection ridgeline between the rake face and the side surface has a cutting edge portion including a main cutting edge located in a side portion of the rake face, a subsidiary cutting edge connected to the main cutting edge, and a corner cutting edge connected to the subsidiary cutting edge and located in a corner portion of the rake face, wherein the side surface includes a flank face and a connection surface which are adjacent to each other in the thickness direction via a boundary line, wherein the flank face is located closer to a rake face side than the boundary line, wherein the connection surface is located closer to a seating surface side than the boundary line, and is located outside an extension line of the flank face in a cross section in a normal direction of a ridgeline of the main cutting edge, and wherein in a cross section in a normal direction of each ridgeline of the main cutting edge, the subsidiary cutting edge, and the corner cutting edge, a passage point of the cutting edge portion is set as a point P, a perpendicular line extending from the point P toward an extension line of the seating surface is set as a line segment H, an intersection point between the line segment H and the extension line of the seating surface is set as a point R, an intersection point between an extension line along the flank face and the seating surface is set as a point Q, a passage point of an intersection ridgeline between the connection surface and the seating surface is set as a point Q', a passage point of the boundary line is set as a point M, a perpendicular line extending from the point M toward the line segment H is set as a line segment N, an intersection point between the line segment N and the line segment H is set as a point M', characterized in that the point Q' is located closer to a point Q side than a point S located at a distance of 1/3 of a line segment QR from the point R, the point M' is located closer to a point R side than a point T located at a distance of 2/3 of the line segment H from the point R, the seating surface and a portion of the flank face which is connected to the main cutting edge function as restriction portions in contact with the tool main body.