A cutting insert has parallel first upper and lower faces, parallel second upper and lower faces, first and second side faces, first and second cutting portions, and first and second mounting grooves. The second upper and lower faces are perpendicular to the first upper and lower faces, respectively. The first and second side faces are located at respective ends of the cutting insert. The first cutting portion has a major cutting edge at an edge between the first upper face and the first side face, while the second cutting portion has a major cutting edge at an edge between the second upper face and the second side face. The first mounting groove curvedly or linearly extends from the first cutting portion oppositely to the second cutting portion, while the second mounting groove curvedly or linearly extends from the second cutting portion oppositely to the first cutting portion.