To measure the accurate length of a cobwebbed yarn by providing a binalization section binalizing an image of the actual space from an inverse transformation section and converting it into a binalized image, and measuring the length based on the binalized image when detecting the cobwebbed yarn on an end face with image pickup means photographing the end face of a yarn package.

A cheese-shaped yarn package 1 to be inspected is conveyed on a tray 3 to an inspection position, the yarn package 1 is rotated by the rotation angle corresponding to a reference yarn length on the innermost periphery of a yarn layer on an end face by a driving means 2 at the inspection position, and the image of each end face section is photographed by a plurality of image pickup means 4. In this cobwebbing inspecting means 5, the photographed image is converted into an image of the frequency space by two-dimensional frequency space processing, and a specific frequency region containing the image of the frequency space of a cobwebbed yarn is extracted from this image. The extracted specific frequency region is inversely converted into an image of the actual space by two-dimensional frequency space processing, and the length of the cobwebbed yarn is measured based on the obtained image.