PURPOSE: To obtain a thin small diameter tube without squarishness, wrinkly flaws on the inner surface of tube and flaws on the outer surface of tube by forming elongating rolling under specific conditions using a plug with a rolling part of which the outside diameter is gradually reduced according to the moving direction of a tubular material as plug.

CONSTITUTION: Inclined rolls 1 and guide shoes are alternately arranged around the X-X line of the pass line of a hollow shell H and rolled tube H' and the plug 2 is situated on the pass center of the hollow shell H and rolled tube H'. In this case, elongating rolling is performed under the following conditions: 0°<β≤16°, Rg≤0.85d, 1.0RG≤G≤1.2Rg, 0°≤θ1-θP≤8°, 0°≤α≤θ1+4°, 0°<θ2≤4°, where, β: tilt angle, d: outside diameter of tubular material before rolling, Rg: the opening degree of inclined roll, G: the opening degree of guide member for tubular material, θ1: the face angle of the inlet of inclined roll θ2: the face angle of the outlet of inclined roll, d: the face angle of the inlet of guide member for tubular material, θP: the face angle of the rolling part of plug.