What is claimed is:
1. An apparatus for detecting the degree of soiling of a sheet, comprising:
means for detecting light having interacted with an entire region of the sheet upon irradiation of the light onto the sheet; signal generating means for generating a non-printed region signal which identifies portions of the sheet which are non-printed regions according to a type of the sheet;
calculating means for calculating an output of said detecting means in accordance with the non- printed region signal and calculating an amount of light having interacted with the non-printed regions of the sheet; and means for comparing the amount of light having interacted with the non-printed regions with non- printed region reference signals, and for judging the soiling of the sheet from the output of said comparing means.