The invention relates to an image processing apparatus and a determination method. A signal value representing at least one of a plurality of types of optical characteristics are calculated for each pixel from the read signal obtained and output by reading light reflected by a document placed on a document table and a document table cover while the document is covered with the cover. It is determined, based on the signal value calculated, whether or not a target pixel is a pixel in a document region. A document region is detected from the determination result.