A preprocessing section binarizes input image data and calculates a total black pixel ratio. A feature extracting section detects connected components included in the binary image data and detects circumscribing bounding boxes of the connected components. Predetermined connected components are removed from all of the connected components based on the sizes of the detected circumscribing bounding boxes and bounding box black pixel ratios. By using the connected components that remain after removing the unnecessary connected components, a histogram is generated by specifying the sizes of the circumscribing bounding boxes as classes and numbers of the connected components as the frequencies of occurrence. A determining section determines whether the input image data is document image data or non-document image data based on information related to the generated histogram and the total black pixel ratio.