PURPOSE: To improve housing strength of a head pipe and manufacture this head pipe at low cost.

CONSTITUTION: Inner pipes 29, 30 are press-fitted into both end parts of a head pipe material 28, and both end parts of the head pipe material 28 are expanded in this press fit condition of the inner pipes to form head pipe housings 26, 27. Otherwise, after roughing by machining the inner circumferential faces of both end parts of the head pipe material 28 or the outer circumferential faces of the inner pipes 29, 30, the inner pipes 29, 30 are press-fitted into both end parts of the head pipe material 28, and succeedingly both end parts of the head pipe material together with the inner pipes 29, 30 are expanded in one body to form the head pipe housings 26, 27.