A cutting tool, comprising: a cutting insert and a holder, the cutting insert comprising a cutting edge, the holder having a rod shape and comprising: a shank portion located at a rear end side of a central axis of the holder; and a head portion located closer to a front end side than the shank portion, the cutting insert being mounted on the head portion, wherein the head portion comprises: an upper jaw portion and a lower jaw portion located at the front end and facing each other in an opposed manner; an insert pocket located between the upper jaw portion and the lower jaw portion, the cutting insert being mounted in the insert pocket in such a manner that the cutting edge projects from one side surface side in a direction orthogonal to the central axis; a first wall surface extending toward the other side surface side from an end portion of the one side surface side on an upper surface of the upper jaw portion, wherein the other side surface side is located at a side opposite to the side from which the cutting edge projects; the first wall surface is inclined upward as the first wall surface extends toward the other side surface side and the cutting tool further comprises a second wall surface located closer to the rear end side than the first wall surface and located at the one side surface side; characterized in that the second wall surface is inclined upward as the second wall surface extends toward the rear end side; wherein the cutting tool further comprises a concave-shaped third wall surface located closer to the rear end side than the second wall surface; and a fourth wall surface located closer to the rear end side than the third wall surface, the fourth wall surface inclined upward as the fourth wall surface extends away from the third wall surface, and the third wall surface is concave downward from an upper end of the second wall surface, and the fourth wall surface is located above the upper end of the second wall surface. a shank portion located at a rear end side of a central axis of the holder; and a head portion located closer to a front end side than the shank portion, the cutting insert being mounted on the head portion, wherein the head portion comprises: an upper jaw portion and a lower jaw portion located at the front end and facing each other in an opposed manner; an insert pocket located between the upper jaw portion and the lower jaw portion, the cutting insert being mounted in the insert pocket in such a manner that the cutting edge projects from one side surface side in a direction orthogonal to the central axis; a first wall surface extending toward the other side surface side from an end portion of the one side surface side on an upper surface of the upper jaw portion, wherein the other side surface side is located at a side opposite to the side from which the cutting edge projects; the first wall surface is inclined upward as the first wall surface extends toward the other side surface side and the cutting tool further comprises a second wall surface located closer to the rear end side than the first wall surface and located at the one side surface side; characterized in that the second wall surface is inclined upward as the second wall surface extends toward the rear end side; wherein the cutting tool further comprises a concave-shaped third wall surface located closer to the rear end side than the second wall surface; and a fourth wall surface located closer to the rear end side than the third wall surface, the fourth wall surface inclined upward as the fourth wall surface extends away from the third wall surface, and the third wall surface is concave downward from an upper end of the second wall surface, and the fourth wall surface is located above the upper end of the second wall surface. an upper jaw portion and a lower jaw portion located at the front end and facing each other in an opposed manner; an insert pocket located between the upper jaw portion and the lower jaw portion, the cutting insert being mounted in the insert pocket in such a manner that the cutting edge projects from one side surface side in a direction orthogonal to the central axis; a first wall surface extending toward the other side surface side from an end portion of the one side surface side on an upper surface of the upper jaw portion, wherein the other side surface side is located at a side opposite to the side from which the cutting edge projects; the first wall surface is inclined upward as the first wall surface extends toward the other side surface side and the cutting tool further comprises a second wall surface located closer to the rear end side than the first wall surface and located at the one side surface side; characterized in that the second wall surface is inclined upward as the second wall surface extends toward the rear end side; wherein the cutting tool further comprises a concave-shaped third wall surface located closer to the rear end side than the second wall surface; and a fourth wall surface located closer to the rear end side than the third wall surface, the fourth wall surface inclined upward as the fourth wall surface extends away from the third wall surface, and the third wall surface is concave downward from an upper end of the second wall surface, and the fourth wall surface is located above the upper end of the second wall surface.