Abstract: A method for manufacturing a camshaft by inserting a hollow shaft 3 made of steel into an axial hole 400 of a cam lobe 4 made of a sintered alloy, by expanding the hollow shaft 3 by inserting a mandrel 1 into the hollow shaft 3, and by joining and fixing the expanded hollow shaft 3 and the cam lobe 4 while avoiding the constriction that might otherwise be caused by the retraction of the mandrel 1 inserted, of the expanded hollow shaft 3. The method includes an expanding step using a diverging long face 200, which is so formed on the leading head of the mandrel 1 as to diverge at an angle of about 8 to 30 degrees with respect to the axis toward the trailing end, and a bulging portion 2 formed to lead from the diverging face 200; and an expanding step using the bulging portion 2 of the mandrel 1 and a converging short face 201 which is formed so as to lead from the bulging portion 2 and to converge at an angle of about 8 to 30 degrees with respect to the axis toward the trailing end.