PURPOSE: To provide an assembling type cam shaft or the manufacturing method of the cam shaft by which a breakage is not caused at the joining part of it when the cam shaft is manufactured by bulge processing using a tubular material having a joining part.

CONSTITUTION: After plural cam pieces 4a-4h are arranged by having specific phase differences at the specific positions in a forming die K consists of an upper die 1 and a lower die 2, the tubular material 7 having the joining part Y is inserted in shaft holes of the cam pieces 4a-4h and a journal piece by using a thrusting member 9, then, the tubular material 7 is positioned at the position L2-L2 (or L4-L4) by using the thrusting member 9 so that the expansion deformation amount of the joining part is minimum when the material 7 is Y bulge-processed and the thrusting member 9 is removed to execute the bulge processing. By such a manufacturing method, the force applied on the joining part Y of the tubular material 7 is made small when the bulge processing is executed and occurrence of a breaking or crack at the joining part Y is prevented.