A cutting insert has an upper abutment portion at an upper surface thereof, and two bottom abutment portions and a stopper portion, at a bottom surface thereof. The two bottom abutment portions define bottom planes, parallel and spaced apart from one another. The upper abutment surface defines an upper plane, forming a first angle with the bottom planes. The cutting insert has a relief surface at a front surface thereof, forming a cutting edge with a rake surface formed at the upper surface. The stopper portion defines a stopper plane, which forms a second angle with a relief plane defined by the relief surface. A reference plane perpendicular to the upper plane, passing through the upper abutment portion, intersects the first and second bottom planes at first and second intersection lines, which are spaced apart from the first and second bottom abutment portions.