1. An image determination apparatus which determines whether inputted image data is document image data or non-document image data, comprising: a binarization process section which converts the inputted image data to binary image data by binarizing the inputted image data, and calculates a total black pixel ratio that is a ratio of a number of black pixels to a number of all pixels in the image data; a feature extracting section which detects connected components that are groups of pixels having a same color in the binary image data and circumscribing bounding boxes of the connected components, and generates a histogram by specifying the sizes of the circumscribing bounding boxes as classes and numbers of the components as frequencies of occurrence; and an image determination section which determines whether the inputted 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; wherein the feature extracting section removes the connected components in the circumscribing bounding boxes of which the sizes are smaller than a predetermined size, from all connected components, further removes the connected components in the circumscribing bounding boxes of which bounding box black pixel ratios are out of a predetermined range, based on the sizes of the circumscribing bounding boxes and bounding box black pixel ratios each of which represents a ratio of the number of the black pixels constituting corresponding connected component in corresponding circumscribing bounding box to the number of all pixels in the corresponding circumscribing bounding box, and generates the histogram by using the remaining connected components.