A cutting insert has an approximately polygonal plate shape with a cutting edge formed on a intersection portion between a rake face and a flank. A chip breaker is formed on the rake face, and a chip-breaker concave portion is defined by an inclined surface which is curved in a concave manner and which is smoothly connected to the bottom surface portion while being gradually inclined toward the bottom surface portion, from a peripheral edge portion on a nose portion side to a bottom surface portion.