PURPOSE: To enable accurately measuring the width of a metal plate by a simple constitution even under an advers environment in hot rolling.

CONSTITUTION: The device is provided with vertical rolls 10A and 10B by which both end parts of the metal plate P are rolled and range finders 12A and 12B having a distance L between them independent from the vertical rolls. A distance S between the rolls under no load is calculated by a range finder for rolls 18 based on outputted signals from a position sensor provided for hydraulic cylinders 14A and 14B and motor-driven screws 16A and 16B when rolling is not executed immediately after rolls are exchanged and a zero point correcting reference value (a) is calculated by a calculating and storing device 22 from the distance L and initial values of diameter of rolls Dao and Dbo inputted by setting inputting device 20. After rolling is begun, the distances La and Lb to the outer surfaces of the vertical rolls measured by the range finders when rolling is not executed are inputted in the device 22, the sum of diameters of rolls (Da+Db) is updated and the width of the metal plate W is calculated using an expression W=L-(La+Lb+Da+Db) and inputted in a display and storing device 24 during rolling.