To easily control the working allowance of the diameter at the junction end part of a large truncated cone mandrel element and a small diameter side truncated cone mandrel element and hardly damage the junction end part while the seaprability from the large truncated cone mandrel element to the small diameter side truncated cone mandrel element is kept good.

At least two mandrel elements 1, 2 have such a dimension that the diameter is enlarged in the same direction over the whole length, and the diameter at the end part of the large diameter part of a small truncated core mandrel element 1 is almost equal to or slightly smaller than the diameter at the end part of the small diameter part of a large truncated cone mandrel element 2 in the adjacent two mandrel elements. Narrow width cylinder parts 5, 6 having the equal diameter to each end part diameter are arranged at the end part of the large diameter part of the small truncated cone mandrel element 1 and at the end part of the small diameter part of the large truncated cone mandrel element 2.