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, the image area extracting section including an image area detecting section for comparing a pixel value of each part of an image of the inputted 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 a type of the inputted image data, and a threshold value changing section for changing the threshold value used in the image area detecting section to one suitable for the type of the inputted image data in accordance with the type judged by the discrimination section.