A cutting insert to be detachably mounted on a tip portion of a tool main body rotatable around a rotation axis (O) by using a fixing member , the cutting insert comprising: a main surface formed in a substantially parallelogram shape in a plan view and having a pair of acute angle corner portions and a pair of obtuse angle corner portions ; a seating surface that faces a side opposite to the main surface ; a side surface that connects the main surface and the seating surface to each other; a corner cutting edge disposed in the acute angle corner portion ; and a main cutting edge and a subsidiary cutting edge which are respectively disposed in a pair of side ridge portions of the main surface located each other across the acute angle corner portion , wherein a mounting hole into which the fixing member is to be inserted penetrates the main surface and the seating surface , wherein the main cutting edge is inclined at an inclination angle equal to or larger than 8° with reference to a plane perpendicular to an axial direction of the mounting hole so as to be closer to a seating surface side in a direction from the acute angle corner portion toward the obtuse angle corner portion , wherein the main surface has a region inclined in a direction away from the seating surface in a direction from the obtuse angle corner portion toward an edge portion of the mounting hole along a diagonal line connecting the pair of obtuse angle corner portions to each other, and wherein in a cross section along the diagonal line connecting the pair of obtuse angle corner portions to each other, the main surface has a projection portion projecting upward from a straight line (B) connecting the obtuse angle corner portion and the edge portion of the mounting hole to each other, characterised in that a foot portion of the projection portion on an obtuse angle corner portion side and a foot portion of the projection portion on a mounting hole side are located on a seating surface side from the straight line (B). a main surface formed in a substantially parallelogram shape in a plan view and having a pair of acute angle corner portions and a pair of obtuse angle corner portions ; a seating surface that faces a side opposite to the main surface ; a side surface that connects the main surface and the seating surface to each other; a corner cutting edge disposed in the acute angle corner portion ; and a main cutting edge and a subsidiary cutting edge which are respectively disposed in a pair of side ridge portions of the main surface located each other across the acute angle corner portion , wherein a mounting hole into which the fixing member is to be inserted penetrates the main surface and the seating surface , wherein the main cutting edge is inclined at an inclination angle equal to or larger than 8° with reference to a plane perpendicular to an axial direction of the mounting hole so as to be closer to a seating surface side in a direction from the acute angle corner portion toward the obtuse angle corner portion , wherein the main surface has a region inclined in a direction away from the seating surface in a direction from the obtuse angle corner portion toward an edge portion of the mounting hole along a diagonal line connecting the pair of obtuse angle corner portions to each other, and wherein in a cross section along the diagonal line connecting the pair of obtuse angle corner portions to each other, the main surface has a projection portion projecting upward from a straight line (B) connecting the obtuse angle corner portion and the edge portion of the mounting hole to each other, characterised in that a foot portion of the projection portion on an obtuse angle corner portion side and a foot portion of the projection portion on a mounting hole side are located on a seating surface side from the straight line (B). <EOS>