A cutting insert including first cutting edges and second cutting edges respectively formed in a first end surface and a second end surface which are opposed to each other; at least one groove formed in a peripheral side surface which connects the first end surface and the second end surface; and, while one end of the groove reaches the first end surface, another end thereof reaches the second end surface, and an opening width in the first end surface is different from an opening width in the second end surface.

A cutting insert including first cutting edges and second cutting edges respectively formed in a first end surface and a second end surface which are opposed to each other; at least one groove formed in a peripheral side surface which connects the first end surface and the second end surface; and, while one end of the groove reaches the first end surface, another end thereof reaches the second end surface, and an opening width in the first end surface is different from an opening width in the second end surface.