To prevent chips from falling in an inside-diameter groove when the inside-diameter groove is formed on the inner peripheral surface of a peripheral wall on which a projection is formed in the circumferential direction.

When a first snap ring groove 105 and a second snap ring groove 106 are formed on the inner peripheral surface of a clutch drum body 100A on the inner peripheral surface of whose cylindrical peripheral wall 102 an internal spline 103 is formed, the first snap ring groove 105 and the second snap ring groove 106 are formed successively by using a grooving tool including a front cutting edge 13 having the width coincident with those of the snap ring grooves 105, 106, and projections 16, 17 for shrinking and deforming the chips 40 shaved out by the front cutting edge. Since the chips 40 shaved out by the front cutting edge 13 is shrunk and deformed by the projections 16, 17 being chip shrinking means, the chips are prevented from falling in the formed snap ring grooves 105, 106.