PURPOSE: To prevent roll slip and to completely eliminate the generation of the collision of a tube between respective stands by executing drooping to the speed of rotation of rolls of a reducer in accordance with the load of an electric welded steel manufacturing line.

CONSTITUTION: In the case of an electric resistanse welded tube 4, drooping is executed to the speed of rotation of the rolls 9 of the reducer 3 in accordance with the load. The amount 8 of load to the whole mill is detected and drooping ratio DR is changed based on an equation by the magnitude of the load. Equation: Drooping raio DR = Rated load/Amount of Load × D. Where, D: The amount of the specified drooping corresponding to the rated load. The corrected speed S of the set speed S1 at a 1st stand by the drooping ratio DR determined by the equation is determined by S = S1 × DR About the stands on and after the 2nd stand, the drooping of the set speed is also similarly executed. In this way, roll slip at each stand is dissolved and the generation of collision of the tube between respective stands is completely eliminated.