To manufacture a camshaft usable as it is even when a finishing and polishing process is omitted by providing the surface characteristic to be requested in advance for a final product of a cam lobe, and reducing in advance the dimension of a base circle part by the expansion of the diameter corresponding to the tensile stress left in the cam lobe after expansion joining.

In a manufacturing process of an assembled shaft by a partial hydraulic expansion method, a cam lobe 2 is freely fitted to a Traeger pipe 1, a probe 3 is inserted therein, a high hydraulic pressure is applied to a freely fitted area of the cam lobe 2 so that the Traeger pipe 1 is partially expanded and elastically deformed, and further plastically deformed and brought into contact with an inner surface of the cam lobe 2, the expansion is further continued to elastically deform the cam lobe 2, the hydraulic pressure is relieved before the plastic deformation is started, and the cam lobe 2 is force-fitted on the Traeger pipe 1. The expansion condition is established by carrying out a preliminary test.