PURPOSE: To correctly calculate an area ratio for every line of an impression by equally dividing the press plate for every predetermined lateral pitch, providing an arbitrary number of memory areas, integrating the number of dots for every area and processing an integrated number and the number of picture elements in the areas.

CONSTITUTION: A press plate area gage comprises memory area establishing means, dot integration means for integrating the number of dots in an area and area ratio calculating means for every area. On the screen 1 of a CRT, receiving an input from a CPU and a video RAM, the image of a desired press plate 2 is formed. The area 3 is equally divided for every predetermined pitch into (n) in the lateral direction of the press plate 2. In this case, a spacing into which the area 3 is equally divided is made the same as the screw width of the ink pot of a printing machine. Then, the area ratio of the area 3 is measured. Therefore, an image area ratio in the area 3 can be obtained by arithmetically processing the number of the whole picture elements in each of the area 3 by the number of bits bit on in the area 3. The result of a calculation is outputted by an output system, such as a paper tape or the like, according to a use.