A throwaway insert for a pin mirror cutter in which eight corners can be used and which the cutting performance and machining accuracy can be increased by making positive the axial rake angle of the peripheral cutting edge. Side faces of a negative insert of a parallelogrammic shape are formed by two surfaces connecting to each other at an angle so as to have a first included angle and a second included angle. Further, independent arcuate surfaces are provided at the intersections between the arcuate corner sides of each of the side faces and the top and bottom surfaces and the intersections between the obtuse corner sides and the top and bottom surfaces.