PURPOSE: To manufacture tubes having good surface quality, dimensional accuracy, and roundness with good productivity by dividing a rolling stage into three stages of reduction, release, and wall forming for elongation and rolling of tubes by a diagonal rolling mill provided with a plug and plural rolls.

CONSTITUTION: A roll 1 (equaling to rolls 2, 3) consists of a reduction roll face 1a, reduction gouge 1e, 1st release roll face 1b, neck 1f, wall reduction roll 1c, wall reduction gouge 1g, and 2nd release roll face 1d. For rolling a tube stock 6a by use of the rolls 1, 2, 3 and a plug 4 in that structure, first, a gap between the reduction gouge 1e, etc., and the plug 4 is nearly equaled to a wall thickness of the stock 6a and a prescribed inside diameter is obtained by mainly reducing the outside diameter. Then, the stock 6a is released and is formed into a nearly round sectional shape by use of the 1st release roll face 1b, etc. Further, a wall is formed to have a prescribed thickness by the wall reduction roll face 1c, etc., and the plug 4; finally, a round sectional shape is finished by the 2nd release roll face 1d, etc., and a product 6b is delivered.