An image forming apparatus includes: an image forming unit for forming a correcting image for correcting gradations of an output image, on a bearing body; a sensor for measuring reflected light quantity of the correcting image formed on the bearing body; a gradation correcting unit for correcting the gradations of the output image, based on a measurement result of the measured reflected light quantity of the correcting image; and a timing correcting unit for detecting a shift of measurement timing, based on the measurement result by the sensor, and for correcting the detected shift of the measurement timing.