PURPOSE: To provide a measurement method allowing the shrinkage rate of a web to be measured at a breadthwise arbitrary position during travel along the drier part of a paper machine.

CONSTITUTION: The image of light transmitted from the felt surface side to the wire surface side for a paper web travelling along the drier part of a paper machine is subjected to two-dimensional Fourier conversion, thereby obtaining a two-dimensional spectral image. Furthermore, the spatial frequency of the high spectral density area of the web in the image due to a wire mark is obtained, and the shrinkage rate of the web is calculated from a difference between the spatial frequency and the preliminarily obtained spatial frequency of a paper machine wire in use.