A cutting insert having a polygonal shape for right angle machining, comprising: opposed upper and lower surfaces ; a peripheral side surface interconnecting the upper surface and the lower surface at outer peripheries of the upper surface and the lower surface ; and at least one main cutting edge and at least one secondary cutting edge alternately formed along the outer peripheries of the upper surface and the lower surface , wherein an adjacent pair of the main cutting edge and the secondary cutting edge form a substantially right angle therebetween, wherein the upper surface and the lower surface are rotationally symmetrical around an axis perpendicular to a central axis of the cutting insert , characterized in that : the peripheral side surface includes: a first side surface adjoining the main cutting edge of the upper surface ; a second side surface adjoining the secondary cutting edge of the lower surface ; a third side surface adjoining the secondary cutting edge of the upper surface ; and a fourth side surface adjoining the main cutting edge of the lower surface ; the first side surface forms a first obtuse angle (α) relative to the upper surface ; the second side surface forms a second obtuse angle (β) less than the first obtuse angle (α) relative to the upper surface ; the third side surface forms a first acute angle (γ) relative to the upper surface ; and the fourth side surface forms a second acute angle (δ) less than the first acute angle (γ) relative to the upper surface . opposed upper and lower surfaces ; a peripheral side surface interconnecting the upper surface and the lower surface at outer peripheries of the upper surface and the lower surface ; and at least one main cutting edge and at least one secondary cutting edge alternately formed along the outer peripheries of the upper surface and the lower surface , wherein an adjacent pair of the main cutting edge and the secondary cutting edge form a substantially right angle therebetween, wherein the upper surface and the lower surface are rotationally symmetrical around an axis perpendicular to a central axis of the cutting insert , characterized in that : the peripheral side surface includes: a first side surface adjoining the main cutting edge of the upper surface ; a second side surface adjoining the secondary cutting edge of the lower surface ; a third side surface adjoining the secondary cutting edge of the upper surface ; and a fourth side surface adjoining the main cutting edge of the lower surface ; the first side surface forms a first obtuse angle (α) relative to the upper surface ; the second side surface forms a second obtuse angle (β) less than the first obtuse angle (α) relative to the upper surface ; the third side surface forms a first acute angle (γ) relative to the upper surface ; and the fourth side surface forms a second acute angle (δ) less than the first acute angle (γ) relative to the upper surface . the peripheral side surface includes: a first side surface adjoining the main cutting edge of the upper surface ; a second side surface adjoining the secondary cutting edge of the lower surface ; a third side surface adjoining the secondary cutting edge of the upper surface ; and a fourth side surface adjoining the main cutting edge of the lower surface ; the first side surface forms a first obtuse angle (α) relative to the upper surface ; the second side surface forms a second obtuse angle (β) less than the first obtuse angle (α) relative to the upper surface ; the third side surface forms a first acute angle (γ) relative to the upper surface ; and the fourth side surface forms a second acute angle (δ) less than the first acute angle (γ) relative to the upper surface . <EOS>