PURPOSE: To roll uniformly the dog bone parts at both ends of a material to be rolled by providing calibers to the vertical rolls positioned at the front and rear ends of at least one horizontal mill of a series of plural reversing roughing- mills and edging the material heavily as well as rolling it slightly in the vertical direction by the horizontal mill.

CONSTITUTION: Plural reversing roughing-mills RU1WRU3, consisting of vertical rolls E1WE4 and horizontal mills R1WR4, and a reversing table 5 are disposed adjacently to each other on the upstream side of heating furnaces 1, and also a delay table 3 is disposed on the downstream side of the line of the same side, so as to connect them to a series 4 of finishing mills. Calibers are provided to the vertical rolls E1WE4 disposed at the front and rear sides of at least one of the horizontal mills R1WR4 of said roughing mills RU1WRU3, for edging the material heavily as well as rolling it slightly in the vertical direction by at least one of the horizontal mills E1WE4.