To rapidly and certainly perform the adjustment of the folding operation timing of a folder, the replacement of a part, the sampling work of inferior goods or the like by detecting the generation of corner folding, rupture or the like easy to generate at the end part of a signature in a line for feeding the signature.

In a signature inspection method for inspecting deficiency such as the corner folding, rupture or the like of the end surface part of the signature subjected to folding processing by a folder to be fed, pressure is applied to both side edge parts of the signature after both side edge parts of the signature are arranged to obtain the pressure distribution from the signature in a signature feed direction and the obtained pressure distribution is analyzed to judge the presence of the deficiency. A folding deficiency detector using this folding deficiency detection method is provided.