PURPOSE: To mass-produce a lightweight product of high quality, by applying the drawing work and the hydraulic bulging work to the blank tube.

CONSTITUTION: The outer surface of middle zone of the blank tube 10 is firmly held by upper and lower holders 30a, b; the pistons 34a, b, press the drawing dies 32a, b, set on outside of both ends of the blank tube 10, toward the middle of the blank tube 10, whose both ends' diameter is hereby reduced; by which a drawn tube 11 is obtained. Next, this drawn tube 11 is set in the segmental bulging dies 40a, b, and the bulging pressure is applied to the inside of the drawn tube 11; at the same time, the bulging dies 40a, b, are axially compressed in order to bulge the middle of the drawn tube 11 into a desired shape corresponding to the dies' cavity; hereby, a bulged tube 13, provided with symmetrical differential gear holding dome and excess metal absorbing dome, is obtained. The excess metal absorbing dome of this bulged tube 13 is cut off the insertion port for the driving mechanism is formed, and besides, a reinforcing plate is welded around the insertion port.