PURPOSE: To generate a density histogram with high accuracy by generating the density histogram from the density data of a picture element in the area of an original based on the shape of the original detected with an original shape detecting means.

CONSTITUTION: When the original is the one in which read data obtained by scanning the original is suppressed to the original mat of halftone color with a binarizing means 2, it is binarization-processed with different two threshold values. On the other hand, when the original is the one supressed to the original mat of black color, a binarization processing is performed with one threshold value, and it is stored in a storage means 3, and is realized with a processing program stored in advance in the memory of a central control part 10, and the shape of the original is detected with the original shape detecting means. And the density histogram is generated from the density data of the picture element in the area of the original with the data processing means 13, 10 based on the detected shape of original. Thereby, a precise density histogram can be generated.