A cutting insert includes a rake face configuring one of polygonal surfaces, a seating surface configuring the other one of the polygonal surfaces, and a side surface connecting the rake face and the seating surface to each other. The cutting insert 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. The side surface includes a flank face and a connection surface which are adjacent to each other in a thickness direction via a boundary line. The connection surface is located on the seating surface side, and is located outside than an extension line of the flank face.