PURPOSE: To measure characteristic values reflecting the condition of an actual image without obtaining a copy actually, by detecting an electrostatic latent image formed on a photosensitive body to measure image characteristics.

CONSTITUTION: Each characteristic value obtained in a signal processing part 50a is outputted to a CPU110 through an interface circuit 60a and is stored temporarily in a work memory 130. A detecting part 30 and a signal processing part 50b have the same constitutions as a detector 20 and the signal processing part 50a respectively, and these two detecting and processing sequences are operated in parallel by a timing controller 70. The CPU110 discriminates the characteristic of each copying machine as an object on a basis of characteristic values stored in the work memory 130 and outputs a corresponding indication or the like from a printer 150. Since the electrostatic latent image is detected to measure image characteristic values in this manner, characteristic values indicating an actually obtained copy picture are obtained.