Abstract: In a corner reduction device for reducing side corners of a slab with corner rolls, a pair of upper and lower corner rolls are mounted inside a unitary unit frame, and a corner roll unit housing the unitary frame which is movable in the direction of width and vertical direction of the material is formed. Vertical position of the corner roll unit is controlled by a lifting power control cylinder through a push rod and position in direction of material width is controlled by a reduction cylinder. Driving of cylinder servo valves is controlled in accordance with position set values set by position setters, load set values set by load setters, and feedback signals from position sensors, thereby controlling the corner roll positions to fixed lateral and vertical positions. The driving of the servo valves is corrected according to values detected by cylinder pressure sensors, to evenly reduce the right and left corners of the material.