An image forming apparatus according to an embodiment of the invention reads an original by mechanically moving an axial light source in a subscanning direction perpendicular to the axis. The image forming apparatus includes: a sensor whose resolution in a main scanning direction parallel to the axis of the axial light source is higher than a designated resolution; a controller that controls the driving speed of the axial light source so that the resolution in the subscanning direction is higher than the designated resolution; an original-image-data generating unit that generates original image data with a resolution higher than the designated resolution in response to the output from the sensor; and an image reducing unit that generates reduced pixels fewer than a specified number of pixels in the original image data to generate a reduced image with a resolution equal to the designated resolution.