PURPOSE: To decrease the number of roll exchanges and the number of the rolls to be possessed by using upper and lower horizontal rolls consisting of plural calibers having different widths and sleeve rolls movable in the axial direction of the rolls for a breakdown rolling mill and executing repeatedly the web rolling and flange rolling of a blank material.

CONSTITUTION: A blank material such as a roughly H-shaped billet (beam blank) or rectangular billet is subjected to hot rough rolling by the breakdown rolling mill. The upper and lower horizontal rolls of the breakdown rolling mill are constituted of plural pieces of the calibers 20, 22, 24, 26 and a pair of the sleeve rolls 28 for each of the upper and lower rolls divided in the direction perpendicular to the roll axis. The sleeve rolls 28 are made variable in position in the axial direction of the rolls. At least ≥1 sets of such upper and lower horizontal rolls are used and the blank material is subjected repeatedly to web rolling and flange rolling.