A flatness detector has a fixed shaft and a plurality of rotary rings loosely fitted over the shaft and adjacent to each other along an axis of the shaft, a pneumatic bearing being defined by a gap between the rings and the shaft. The shaft is provided with heating means for heating the shaft to prevent shrinkage of a rolled strip due to temperature variations of the strip as well as bending means at opposite ends of the shaft for bending the shaft in a predetermined direction so as to prevent formation of lengthwise wrinkles on the rolled strip.