PURPOSE: To enable to adjust an ununiform thickness of a covering for an innermost layer with a simple operation by a method wherein a nipple holder cylinder and a nipple holder having hollow holes eccentric in parallel with respect to a die center line are provided in a specially arranged condition.

CONSTITUTION: A nipple holder cylinder 6 having a hollow hole 61 eccentric in parallel with respect to a die center line L1 is mounted rotatably about the center line L1 while a nipple holder 7 having a hollow hole 71 eccentric in parallel with respect to a center line L2 of the hollow hole 61 is provided rotatably about the center line L2. The nipple holder 7 is rotated so that a nipple center line L3 and the die center line L1 are aligned, thereafter, a preliminary extruding is effected. An amount of an ununiform thickness thereof is investigated and the nipple holder 7 is rotated so that a location of the center line L3 is eccentric with respect to the center line L1 by e', subsequently, the nipple holder cylinder 6 is roatated to align directions of the ununiform thicknesses so that a location of the hollow hole 8 will set off the amount of eccentricity e', thereby, the location of the hollow hole 8 is adjusted so as to locate perfectly concentric with a covering layer 9.