A cutting tool secured by a first end to a rotary drive device and rotated about an axis by the rotary drive device to cut a workpiece, comprising: a shaft-like insert holder having the first end and a second end at the opposite ends; and a plurality of inserts detachably held in the insert holder, wherein the insert holder includes a body having a plurality of grooves formed on an outer circumferential surface, the grooves being closed on the first end side, and open on the second end side, and a lid to be attached to the second end side of the body, the inserts include base portions to be inserted into the grooves, and cutting portions that cut the workpiece, the cutting portions sticking out from the outer circumferential surface of the insert holder while the base portions are held in the grooves, the body includes first projecting portions that are spaced a predetermined distance apart from groove bottoms of the grooves and project from the first end side of the grooves toward the second end, the lid includes second projecting portions that, when the lid is attached to the body, are spaced a predetermined distance apart from the groove bottoms, and project toward the first end, the base portions include first retaining portions that are interposed between the groove bottoms and first projecting portions, and second retaining portions that are interposed between the groove bottoms and second projecting portions, the first projecting portions have first projecting faces on the side of the first retaining portions, the first projecting faces being angled toward the groove bottoms as they extend toward the first end, the second projecting portions have second projecting faces on the side of the second retaining portions, the second projecting faces being angled toward the groove bottoms as they extend toward the second end, the first retaining portions have first retaining faces making surface-contact with the first projecting faces, the second retaining portions have second retaining faces making surface-contact with the second projecting faces, characterized in that: the inserts held in the insert holder reach the second end, the lid has an outer diameter smaller than that of the body, the second projecting portions project into the grooves, the second projecting portions are continuously formed in the circumferential direction of the insert holder, and the body has a plurality of recesses into which the second projecting portions are inserted, and the recesses being formed to connect the grooves. a shaft-like insert holder having the first end and a second end at the opposite ends; and a plurality of inserts detachably held in the insert holder, wherein the insert holder includes a body having a plurality of grooves formed on an outer circumferential surface, the grooves being closed on the first end side, and open on the second end side, and a lid to be attached to the second end side of the body, the inserts include base portions to be inserted into the grooves, and cutting portions that cut the workpiece, the cutting portions sticking out from the outer circumferential surface of the insert holder while the base portions are held in the grooves, the body includes first projecting portions that are spaced a predetermined distance apart from groove bottoms of the grooves and project from the first end side of the grooves toward the second end, the lid includes second projecting portions that, when the lid is attached to the body, are spaced a predetermined distance apart from the groove bottoms, and project toward the first end, the base portions include first retaining portions that are interposed between the groove bottoms and first projecting portions, and second retaining portions that are interposed between the groove bottoms and second projecting portions, the first projecting portions have first projecting faces on the side of the first retaining portions, the first projecting faces being angled toward the groove bottoms as they extend toward the first end, the second projecting portions have second projecting faces on the side of the second retaining portions, the second projecting faces being angled toward the groove bottoms as they extend toward the second end, the first retaining portions have first retaining faces making surface-contact with the first projecting faces, the second retaining portions have second retaining faces making surface-contact with the second projecting faces, characterized in that: the inserts held in the insert holder reach the second end, the lid has an outer diameter smaller than that of the body, the second projecting portions project into the grooves, the second projecting portions are continuously formed in the circumferential direction of the insert holder, and the body has a plurality of recesses into which the second projecting portions are inserted, and the recesses being formed to connect the grooves. the inserts held in the insert holder reach the second end, the lid has an outer diameter smaller than that of the body, the second projecting portions project into the grooves, the second projecting portions are continuously formed in the circumferential direction of the insert holder, and the body has a plurality of recesses into which the second projecting portions are inserted, and the recesses being formed to connect the grooves.