A cutting insert having a polygonal shape for right angle machining, comprising: opposed upper and lower surfaces (22, 24); a peripheral side surface (26, 27, 28, 29) 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 (22, 24); a peripheral side surface (26, 27, 28, 29) 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.