An image forming apparatus comprises a image reader unit and a controller. The controller is configured to i) execute a detection operation of calculating a correction coefficient based on reference image data obtained by applying the light from a light source to a reference unit and receiving, by a photoelectric conversion unit, the reflected light from the reference unit to perform photoelectric conversion of the received light to the reference image data, ii) correct image density variations of the image data caused by orientation variations of the light source and pixel sensitivity of the photoelectric conversion unit based on the correction coefficient, iii) form an output image to be printed on output paper in accordance with an arrangement manner, based on the corrected image data, and iv) determine an interval at which the detection operation is executed, in accordance with a characteristic of the light source and the arrangement manner.