An image processing apparatus includes circuitry to: obtain a read image read from an image of a document output from an image reading device; detect an edge between a background area and a document area in the read image; sample a detection result of the edge at each first interval in a first direction to extract a first boundary point group including a plurality of boundary points; sample the detection result of the edge at each second interval in the first direction to extract a second boundary point group including a plurality of boundary points; determine a noise removal condition based on the first boundary point group; remove, from the second boundary point group, a boundary point satisfying the noise removal condition as noise; and calculate a linear equation approximating the second boundary point group from which the noise is removed.