A cutting insert comprising a first surface, a second surface opposite to the first surface, and a side surface connecting the first surface and the second surface to each other, characterized in that: an intersecting ridge line between the first surface and the side surface includes first and second substantially arcuate ridge lines opposite to each other, the first and second arcuate ridge lines forming first and second arcuate cutting edges (8, 10), respectively; an intersecting ridge line between the second surface and the side surface includes a third substantially arcuate ridge line, the third arcuate ridge line forming a third arcuate cutting edge; when a line drawn at a middle point between the first and second arcuate cutting edges (8, 10) is defined as a middle line, one of the first and second arcuate cutting edges (8, 10) and the third arcuate cutting edge are located at a side surface on one side of the middle line whereas the other of the first and second arcuate cutting edges is located at a side surface on the other side of the middle line; the length of a line segment substantially perpendicular to a tangent drawn on a most expanded portion of each of the first and second arcuate cutting edges (8, 10) is greater than that of a line segment substantially perpendicular to a tangent drawn on each of a most expanded portion of the third arcuate cutting edge and a most expanded portion of an intersecting ridge line between the second surface and the side surface, the intersecting ridge line being opposite to the third arcuate cutting edge; and as viewed in a cross section substantially perpendicular to the middle line, an angle (α) formed between the first surface and a straight line which connects the one of the first and second arcuate cutting edges (8, 10) to the third arcuate cutting edge is different from an angle (β) formed between the first surface and a straight line which connects the other of the first and second arcuate cutting edges (8, 10) to the intersecting ridge line between the second surface and the side surface. an intersecting ridge line between the first surface and the side surface includes first and second substantially arcuate ridge lines opposite to each other, the first and second arcuate ridge lines forming first and second arcuate cutting edges (8, 10), respectively; an intersecting ridge line between the second surface and the side surface includes a third substantially arcuate ridge line, the third arcuate ridge line forming a third arcuate cutting edge; when a line drawn at a middle point between the first and second arcuate cutting edges (8, 10) is defined as a middle line, one of the first and second arcuate cutting edges (8, 10) and the third arcuate cutting edge are located at a side surface on one side of the middle line whereas the other of the first and second arcuate cutting edges is located at a side surface on the other side of the middle line; the length of a line segment substantially perpendicular to a tangent drawn on a most expanded portion of each of the first and second arcuate cutting edges (8, 10) is greater than that of a line segment substantially perpendicular to a tangent drawn on each of a most expanded portion of the third arcuate cutting edge and a most expanded portion of an intersecting ridge line between the second surface and the side surface, the intersecting ridge line being opposite to the third arcuate cutting edge; and as viewed in a cross section substantially perpendicular to the middle line, an angle (α) formed between the first surface and a straight line which connects the one of the first and second arcuate cutting edges (8, 10) to the third arcuate cutting edge is different from an angle (β) formed between the first surface and a straight line which connects the other of the first and second arcuate cutting edges (8, 10) to the intersecting ridge line between the second surface and the side surface.