PURPOSE: To detect the abnormal part of the profile of a belt shaped body accurately without receiving the effect of temporary noises at the time of measurement, by decomposing the measured value at each point in the width direction into frequency components by Fourier transformation.

CONSTITUTION: A physical quantity A(i) (i=1WN) of each point of a belt shaped body 10 in the width direction, e.g., a point (n), is measured at each point (e.g., a point N) in the longitudinal direction. Then, the measured value xj (j=1Wn, where n is power of 2 in general) at each point in the width direction is decoposed into frequency components ak and bk by Frourier transformation by using an expression 1. The frequency component B(i) that is obtained in this way indicates the frequency component of 2πk/n of the original curve. The parts, where (k) is small, medium, and large, are low, medium, and high frequency parts, respectively. Then, only the medium frequency region, wherein an abnormal protruded part to be detected is present, in the frequency component B(i) undergoes inverse Fourier transformation. Thus the abnormal protruded part is extracted.