To reduce an outer diameter or a wall thickness by rolling a cylindrical hollow body, which is fed into an Assel mill from a front part, by approaching a prereducer roll to the hollow body at a specified adjusting speed to be slowly and continuously adjusted.

A cylindrical hollow body 2 fed into an Assel mill from a front part is rolled by the Assel mill, a prereducer 4 reducing a tail end diameter, to which a mandrel bar 4 is inserted, and/or a thickness of the hollow body 2 is arranged in front of the Assel mill as viewing from the inlet direction, a roll of the prereducer is position adjustable for the hollow body 2 to be guided to approach/separated to the hollow body 2, the hollow body 2 fed into the Assel mill from a front part is rolled to reduce the outside diameter or wall thickness. At this time, the prereducer roll 3 is adjusted at an adjustment speed, as taking DB for the outside diameter of the hollow body 2, while the axial direction distance LNEL of the region, where the prereducer roll 3 acts, satisfies a formula I, preferably a formula II.