1. An image processing apparatus comprising an image area extracting section for identifying and extracting, on a basis of inputted image data obtained by reading a document with use of an image input apparatus, an image area within the document where an image is present, a resolution converting section for carrying out resolution conversion on the inputted image data so that converted image data obtained by the resolution conversion has resolution suitable for detection processing, the image area extracting section including an image area detecting section for comparing a pixel value of each part of an image of the converted image data with a threshold value so as to detect, as the image area, an area where a pixel value is larger than the threshold value, and the image area extracting section further including a discrimination section for judging whether the inputted image data is binary image data or multi-level image data, and a threshold value changing section for changing, in a case where the inputted image data is binary image data, the threshold value to a value larger than that in a case where the inputted image data is multi-level image data.