A cutting insert has an upper surface, a lower surface and a lateral surface. The upper surface has a first upper side, and a second upper side. The lower surface has a first lower side and a second lower side. The lateral surface has a first lateral surface, a second lateral surface, and a third lateral surface. The first lateral surface has a first concave part having a first constraining surface having a flat shape. The second lateral surface has a second concave part having a second constraining surface having a flat shape. The third lateral surface has a convex part. The first concave part and the second concave part are separated from each other by the convex part.

A rotary cutting tool, which is excellent in defect resistance and has a stable lifespan, includes a rake surface composed of one surface, a cutting edge angle that is larger than 0 and smaller than 90, a true rake angle that is not smaller than 42 and not larger than 13, and a major cutting edge inclination angle that is not smaller than 5 and smaller than +5. A wiper insert is preferably formed in a rounded convex shape.