1. An image processing apparatus comprising: a reading unit configured to perform a reading process for reading an area including an image of an original document and for generating data; a first determination unit configured to determine whether the image of the original document is a color image or a monochrome image based on the generated data of a first area corresponding to a first size read in the reading process performed by the reading unit; a second determination unit configured to determine whether the image of the original document is a color image or a monochrome image based on the generated data of a second area corresponding to a second size read in the same reading process performed by the reading unit, the second size being different from the first size; a specification unit configured to specify a size of the original document after determining by the first determination unit and determining by the second determination unit; and an adopting unit configured to adopt, in a case where the size of the original document specified by the specifying unit is the first size, a first determination result which has been determined by the first determination unit without adopting a second determination result which has been determined by the second determination unit, and to adopt, in a case where the size of the original document specified by the specifying unit is the second size, the second determination result without adopting the first determination result.