In a stain determination apparatus or sheet processing apparatus, an input processing unit inputs image information containing an inherent variation that is not related to a stain, a feature extracting unit extracts a plurality of feature information items from the image information input by the input processing unit, a separating unit separates a set of the plurality of feature information items extracted by the feature extracting unit into a inherent variation component and another residual component, a determining unit extracts a main component of stain variation indicating a stain degree in the image information input by the input processing unit from the residual component separated by the separating unit and determines a stain degree in the image information based on the magnitude of the extracted stain variation main component.