To shorten a line length by rotation-driving a peripheral speed of roll rows substantially to conform it to a peripheral speed of a slitting cutter, and forming a loop respectively in a steel plate strip and a slit material in an in/out side of the slitting cutter.

As a system detecting respective loop height by loop height detecting sensors 7a, 7b, a rotation drive speed of each driving gear 3a, 4a, 5a can be controlled by a controller 6. For instance, when a loop height between a slitting cutter 3 and a deflector roll 5 is decreased, that is, approximated to a linear path, in the case of control with a peripheral speed of the slitting cutter 3 serving as the reference, by decreasing a peripheral speed of the deflector roll 5, a loop is deepened, and a slit material 23 is maintained in a condition with tension left as zero.