A cutting tool includes a cutting insert that is held in a holder in a detachable manner. The cutting insert has a substantially regular quadrangular shape in plan view and includes four round corners. The cutting insert is formed of CBN sinter as a whole. The cutting insert includes two principal surfaces that are upper and lower surfaces opposite to each other, and four side surfaces that are arranged so as to connect the principal surfaces to each other. A boundary portion (ridge line portion) between each of the principal surfaces and each of the side surfaces forms a cutting edge. In a central portion of each of the side surfaces of the cutting insert, a groove for absorbing a crack, having a substantially V-shape in cross section, is formed all around the cutting insert so as to be recessed with respect to the side surfaces.