A cutting insert having an end cutting edge and side cutting edges extending rearward from opposite ends of the end cutting edge, wherein a rake face of the cutting insert includes a positive rake face inclined downward from the end cutting edge toward the rear of the cutting insert so that a positive rake angle is obtained and further includes a breaker wall rising obliquely from a rear end of the positive rake face or from a position rearward of the rear end, wherein, in a portion of the rake face, the portion extending from the end cutting edge along the side cutting edges in a forward-rearward direction, inclined surfaces (51, 52) are formed such that the inclined surfaces (51, 52) incline downward from the side cutting edges toward a central portion of the rake face between the side cutting edges (120, 120), and a recessed groove located downward of the inclined surfaces (51, 52) is formed in the central portion between the inclined surfaces (51, 52) such that the recessed groove extends rearward from the end cutting edge in the forward-rearward direction beyond the upper end of the breaker wall into a portion rearward of the breaker wall, wherein the recessed groove is formed such that, in a forward-rearward region between the end cutting edge and the upper end of the breaker wall, a groove width of the recessed groove as viewed from a rake face side gradually increases from the position of the end cutting edge toward the rear, then gradually decreases such that the recessed groove has a narrow portion at a position forward of the position of the upper end of the breaker wall and rearward of a positive rake angle rear end position which is the rear end of the positive rake face, characterized in that the groove width of the recessed groove then gradually increases from the position of the narrow portion toward the rear until the position of the upper end of the breaker wall, wherein the recessed groove is formed such that, when the groove width at the position of the end cutting edge is denoted by W1, the groove width at the position of the narrow portion is denoted by W2, and the groove width at the position of the upper end of the breaker wall is denoted by W3, then the groove widths W1, W2, and W3 satisfy a dimensional relation W1 < W2 < W3. wherein a rake face of the cutting insert includes a positive rake face inclined downward from the end cutting edge toward the rear of the cutting insert so that a positive rake angle is obtained and further includes a breaker wall rising obliquely from a rear end of the positive rake face or from a position rearward of the rear end, wherein, in a portion of the rake face, the portion extending from the end cutting edge along the side cutting edges in a forward-rearward direction, inclined surfaces (51, 52) are formed such that the inclined surfaces (51, 52) incline downward from the side cutting edges toward a central portion of the rake face between the side cutting edges (120, 120), and a recessed groove located downward of the inclined surfaces (51, 52) is formed in the central portion between the inclined surfaces (51, 52) such that the recessed groove extends rearward from the end cutting edge in the forward-rearward direction beyond the upper end of the breaker wall into a portion rearward of the breaker wall, wherein the recessed groove is formed such that, in a forward-rearward region between the end cutting edge and the upper end of the breaker wall, a groove width of the recessed groove as viewed from a rake face side gradually increases from the position of the end cutting edge toward the rear, then gradually decreases such that the recessed groove has a narrow portion at a position forward of the position of the upper end of the breaker wall and rearward of a positive rake angle rear end position which is the rear end of the positive rake face, characterized in that the groove width of the recessed groove then gradually increases from the position of the narrow portion toward the rear until the position of the upper end of the breaker wall, wherein the recessed groove is formed such that, when the groove width at the position of the end cutting edge is denoted by W1, the groove width at the position of the narrow portion is denoted by W2, and the groove width at the position of the upper end of the breaker wall is denoted by W3, then the groove widths W1, W2, and W3 satisfy a dimensional relation W1 < W2 < W3.