PURPOSE: To provide a plate width detecting device and a controller for a vertical mill which can be used to realize higher-precision plate width control in the vertical mill.

CONSTITUTION: A fluctuating component ΔPW of a rolled load due to fluctuation of the plate width on the inlet side of the mill and a fluctuating component ΔPQ of the rolled load due to temperature fluctuation of a rolled stock are detected respectively individually from a plate width WI on the inlet side, a rolled load P and a roll gap S and further an individual control gain is set to a fluctuating component of each rolled load to control the roll gap. Consequently, a high-precision control for the plate width can be realized under individual consideration of influences of fluctuation AS of the roll gap, fluctuation ΔWI of the plate width on the inlet side of the mill and temperature fluctuation ΔT of the rolled stock.