A cutting insert to be detachably mounted on a tip portion of a tool main body rotatable around a rotation axis 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 (6a, 6b) 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 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. 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 (6a, 6b) 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 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.