PURPOSE: To accurately measure a printing area ratio without any influence of the irregular density of a printing plate, and the deflection, warping, fine irregularities or the like of the plate.

CONSTITUTION: An image pickup means 7 is used to divide a printing plate 1 into unit photographing areas, and resolve the areas into a picture element sufficiently smaller than a minimum net point for taking the photograph thereof. Then, an image signal obtained therefrom for every picture element is sent to an area computing means. This means performs pre-treatment including a shadow border line correction for an illuminant irregularity and edge highlighting or the like. In addition, the means performs a binarization process, using the prescribed threshold value. The means after obtaining a binarized image signal, sends the signal to a printing area ratio computation means, and this means counts the number of picture elements contained in the image to calculate the area of a printing area section or non-printing area section, thereby providing a printing area ratio for every photographing unit area.