A cutting insert has an upper face, a lower face, a side face, an upper cutting edge, a lower cutting edge and a through hole. The upper face has a substantially polygonal shape. The side face has a first side face and a second side face. The first side face has at least one upper concave part and at least one lower concave part. The at least one upper concave part and the at least one lower concave part are separated from each other through a thick part being continuous along a width direction of the side face. The thick part has a curved region extending in the shape of a wave from one end to the other end in the width direction of the first side face in a side view.