To provide a method for manufacturing an assembly shaft capable of reducing manufacturing cost by the decrease of man days, as a method for manufacturing an assembly shaft such as an assembly camshaft.

An outer circumferential surface 2A of a hollow shaft 2 is plastically deformed for its diameter to be expanded into a deformed cross sectional shape similar to the cross sectional shape of a specified plug, by way of a specified plug penetrating through along an inner surface 2B of the hollow shaft 2, while an outer circumferential surface of an end piece 3 is also plastically deformed for its diameter to be expanded into a deformed cross sectional shape similar to the cross sectional shape of the outer circumferential surface 2A of the hollow shaft 2. As a result, the outer circumferential surface of the hollow shaft 2 is brought to have the end piece 3, cam lobes 4, and journals 5 integrally and tightly attached thereto, while the outer circumferential surface of the end piece 3 has end piece flanges 6 integrally and tightly attached.