To execute the double face printing which does not impart a sense of incongruity to a user by preventing "strike-through".

The image signal of an original manuscript 1 equivalent to two pages of the front and the rear is read by an image reading part 10. It is outputted to a control part 90. A/D conversion is executed to the inputted image signal. After that, normal image processing is applied. The image data are generated. They are stored in an image memory 92. Successively, the image data of the region equivalent to hundred lines are read out. The occupying percentage of the black data number to all data number is calculated as an image density A in an image density detecting part 94. If all the image density A of the image data of the front/rear faces stored in the image memory 92 is below 30 %, the image data are printed with a normal print density since they are mostly composed of characters. If the image density A exceeds 30 % at least in one region of the image data, the print density is reduced by generating stencil printing data by applying thinning-out processing as print density reduction processing to the image data of the front/rear faces since a solid part is included.