PURPOSE: To elevate the measuring accuracy, by correcting an error of the camber amount due to rotation of a plate material, when the camber amount is calculated by picking up plural plate material partial images reflected on a polyhedral mirror, by an image pickup machine.

CONSTITUTION: An image pickup machine 13 picks up band areas 301W308 being a partial image of a plate material 30 reflected on a polyhedral mirror 11. This video signal is processed by a binary-coding circuit 15, and after that, is supplied to an operation processor 16. Subsequently, the processor 16 executes a prescribed operation in accordance with position data 30l, 30r of the side end edge of each band area 301W308, and calculates the camber amount of the plate material 30. In this case, the extent of displacement in the same time of the position data of 30l, 30r is derived, and from this extent of displacement, each position data is corrected. As a result, an error of the camber amount by rotation of the plate material 30 is corrected. In this way, accuracy for measuring the camber amount can be elevated.