There is provided a cutting insert which is capable of cutting by using either one of both surfaces and is stably supported by a tool body. Each of a first end surface and a second end surface of a cutting insert includes a first inclined surface which is inclined from a diagonal line by which a corner portion connecting a first peripheral side surface and a second peripheral side surface and a corner portion connecting a third peripheral side surface and a fourth peripheral side surface are joined toward a corner portion connecting the first peripheral side surface and the fourth peripheral side surface such that a distance to an imaginary plane decreases, and a second inclined surface which is inclined from the diagonal line toward a corner portion connecting the second peripheral side surface and the third peripheral side surface such that a distance to the imaginary plane decreases.