Throwaway insert and throwaway-type rotary cutting tool

There is disclosed a insert and a rotary cutting tool which can obtain a machined surface with high precision and high quality, and can improve chip separation to perform higher speed cutting. A main cutting edge 7 extends rearward in a rotating direction of a tool body as the main cutting edge approaches a rear and of the tool body at the outer periphery of the tool body. The main cutting edge is formed in a shape of a convexly curved line which protrudes toward the outside of a rake face 2 as seen from the top and a shape of another convexly curved line which extends toward the seat face while being convexly curved from one end 7A of the main cutting edge to the other end 7B thereof as seen from the side facing the relief face. With a insert mounted to the tool body, an inclination angle defined by a rake face 2 gradually decrease from one end 7A of the main cutting edge toward the other end 7B thereof, and a projection line of a rotation locus of then main cutting edge 7 about a rotation axis of the tool froms a shape of a convex arc which smoothly comes, at a middle portion 7C of the main cutting edge, into contact with a cylindrical surface P about the axis and which gradually retreats inward from the cylindrical surface P as it approaches both ends 7A and 7B of the main cutting edge.