The cutting insert is provided with a polygonal-shaped insert main body (2) having two polygonal surfaces (3, 3) spaced away from each other and a plurality of side surfaces formed between these polygonal surfaces and also provided with cutting edges (8A, 8B, 8C) formed as a rake face on a peripheral edge of a first side surface (4) among the plurality of side surfaces. Among the plurality of side surfaces, a second side surface (6) which is positioned on the opposite side of the first side surface and given as a seating surface in contact with an insert seat of the tool main body for attaching the cutting insert is formed in a V-shape when viewed from the thickness direction of the insert main body and includes two contact portions (6A, 6A), each of which is formed in a raised curve shape on a cross section along the thickness direction, and the two contact portions are formed in a recessed V-shape so as to be greater in depth moving toward the center of the second side surface on the cross section along the thickness direction.