An image processing apparatus includes a reading device and at least one processor. The reading device reads a document into image data of a document image with a first resolution and a second resolution that are different from each other. The first resolution is used for reading the document in a main scanning direction. The second resolution is used for reading the document in a sub-scanning direction perpendicular to the main scanning direction. One or more processors detect an inclination angle of the document image corresponding to inclination of the document, obtain a distortion angle of the document image based on the inclination angle and a ratio between the first resolution and the second resolution, and correct distortion of the document image using the distortion angle.