An image processing apparatus acquires a plurality of first images and one or a plurality of second images. The first images are acquired through a first reading operation in which a reading apparatus sequentially reads first sides of a plurality of document sheets. The second images are acquired through a second reading operation in which the reading apparatus reads second sides of one or more of the plurality of document sheets. In a case where the number of second images acquired is smaller than the number of first images acquired, the image processing apparatus determines a sequence of the plurality of first images and the one or the plurality of second images such that sequential numbers of the one or the plurality of second images are differently set depending on whether an interrupting factor has arisen.