To obtain a high precision steel by measuring a camber quantity again after straightening a camber by a mill roll device based on a camber measured value in the sheet width direction of a slit stock in which a steel sheet supplied from a coil is flattened by a leveler and is cut in a prescribed length and conduct again camber straightening when exceeding a allowable range.

A steel sheet supplied from a steel roll 1 is straightened for coiled propensity by a leveler 3 to flatten and is cut to a slit stock by a shearing device 4. The slit stock is subjected to camber straightening based on a camber quantity measured by a measuring instrument 9 by adjusting a reduction force at a short length side in the longitudinal direction of the slit stock in the mill device 6. When a camber quantity measured again by a measuring instrument 9' exceeds an allowable range, the slit stock is subjected to straightening by passing the mill device 6 again. A controller 200 is an automatic arithmetic unit of a camber quantity. By this method, a minileveler for camber straightening is obtained.