This cutting insert is provided with a rake face, a flank, and a cutting edge. The cutting edge has: a bottom cutting edge which is located at the tip in the direction of the center axis and has a protruding arc shape; and an outer peripheral cutting edge which is continuous with the outer edge in the radial direction of the bottom cutting edge and has a protruding arc shape having a radius of curvature greater than the bottom cutting edge. The angle formed between the center axis and the tangent to the boundary point between the bottom cutting edge and the outer peripheral cutting edge is less than 45°. The radius of curvature R1 of the bottom cutting edge is within the range of 0.3 to 10 mm. The ratio (R2/R1) of the radius of curvature R2 of the outer peripheral cutting edge to the radius of curvature R1 of the bottom cutting edge is within the range of 3.6 to 333.