PURPOSE:To enable finding of an accurate camber amount at a less installation cost, by a method wherein 3 or more measuring points are set along a reference line in parallel to an advancing direction of a laminar material, a plate width gauge is mounted at a reference point at an uppermost stream side, and an edge distance gauge at a reference point at a downstream side. CONSTITUTION:A plate width gauge 30 is mounted at a measuring reference point C at an uppermost stream side of a reference line A in parallel to an advancing direction of a laminar material 14 to detect a distance x(t) from a plate width W0(t) and the reference line A to one edge of the laminar material 14. Edge distance gauges 32 and 34, installed along the reference line A at given intervals l0 and reference points D and E at the one edge of the laminar material 14. The detecting signals are inputted to memories 40, 42, 44, and 46 in synchronism with an instruction of a timing instruction device 38, and the storing values are operated via a plate width processing circuit 50, and plate width central position computing circuits 52, 54, and 56 to find an accurate camber amount. Only one plate width gauge 30 is used, and this reduces an installation cost.