PURPOSE: To save space and to improve the accuracy in trimming by following up the meandering of a belt like plate just before a side trimmer, revolving the side trimmers provided at the both sides thereof, and moving with its perpendicularly intersecting a line.

CONSTITUTION: When an approaching steel plate S causes a centering slippage, the meandering δ is detected by an edge sensor 17, input to a central arithmetic unit, a motor 3 for shifting the whole body is driven and a slide base 2 is slided in the meandering direction. A rotary cutter 16 is then moved to the meandering position of the steel plate S, a signal of the slippage δ and steel plate speed is input to the central arithmetic unit, the arithmetic result is given to the motor 3 for shifting and a motor 14 for steering as well and they are driven according to the signal thereof. The cutting plate width is changed a little when a casing 10 is revolved on the horizontal face and so a motor 9 for adjusting plate width is driven according to the revolving amount thereof and the plate width changing amount is corrected.