To improve threadability by providing pinch rolls capable of effecting one side reduction between a slitter and a tension reel, controlling the rolling force of pinch rolls on the drive side and the work side, and straightening the strip in the direction of keeping the strip apart from the center line.

Pinch rolls 7 nondriven and capable of adjusting one side reduction are provided on the outlet side of a slitter 2. At the time of threading, only the vicinity of a center part of a steel sheet 5 of the work side WS is subjected to reduction, and a tensile strength distribution δ (x) due to frictional force between the steel strip 5 and the pinch roll 7 is generated; thereby, giving a clockwise angular moment M on the strip 5, the strip is moved from the line-center to the work side WS. Further, the amount of correction of deviation between the line-center and the strip 5 is controlled through the optimum adjustment of rolling force on the strip 5 by the pinch roll 7 capable of adjusting one side reduction.