To separately manufacture products having plural sizes to one size of inlet- side dimensions and to reduce the changing time of rolling rolls and number of reserved rolls by adjusting the actual reduction of area at a 1st pass within a specific range in accordance with the diameter of the target product and adjusting an interval between the guide rollers on the inlet side of a 2nd pass in accordance with the increase and decrease of the actual reduction of area.

With a block mill in which two-roll stands #1-#4 on the inlet side of which the guide rollers 2a-4a are continuously arranged shifting a screw-down direction at 90° to each other, a round steel bar is manufactured. When the reference reduction of area of the 1st pass with the rolling rolls 1a of the stand #1 is expressed by R1B(%) and the actual reduction of area by R1=(R1B±X)(%), the actual reduction R1 of area at the 1st pass is adjusted within the range of >2 to ≤10% in X in accordance with the diameter of the target product and also the interval between the guide rollers 2b which are arranged on the inlet side of the stand #2 (the 2nd pass) is adjusted in accordance with the increase and decrease of the actual reduction R1 of area. In this way, the number of the reserved rolls is reduced and the series of rolls can be integrated.