PURPOSE: To prevent unstable detection due to deviation in the printing position by a method wherein a medium is irradiated with light and reflected light is inputted into an image sensor whose output is inputted into a CPU to perform computation for setting brightness for paper surface, arithmetic mean of print density and comparison with the reference value.

CONSTITUTION: Light from a light source 14 is irradiated on a pass book 1 and reflected light is received with an image sensor 12 through a lens 22. The output thereof is digitized with an A/D converter 15 and memorized into an RAM16. The brightness of the paper surface is detected in the subscanning direction 19 and the data is memorized into a CPU17. Then, only the printing part is selected to remove stain or the like of a paper and the print density is compared with the reference density of an ROM20, computing arithmetic mean by the CPU17. Then, the density signal is outputted to an output circuit 21.