The data conversion unit of an image forming apparatus converts input image data into dot image data in which pixel values having gradation information are arranged in a matrix. A storage unit stores optical face tangle error information of each reflecting surface of a rotary polyhedron. An assignment unit assigns a number to each of the plurality of reflecting surfaces, specifies by which of the plurality of reflecting surfaces light emitted for each line of the dot image data is to be reflected and assigns the number of the reflecting surface to each line of the dot image data. A determination unit determines a light emission mode corresponding to the dot image data based on the optical face tangle error information and the number of the reflecting surface. An exposure control unit causes the light source to emit light in accordance with the determined emission mode.