1. An image forming apparatus comprising: an image carrier that comprises: a central region on which a target image as an object of an image forming is formed, and a peripheral region placed outward of the central region; an image forming unit that forms: a first reference image on the peripheral region and a second reference image on the central region, before the image forming is started, and the target image as the object on the central region based on an image forming condition preset for the image forming and a third reference image on the peripheral region, during the image forming; a measurement unit that measures a physical quantity of the first reference image, the second reference image and the third reference image; and a controller that corrects the image forming condition based on the physical quantity measured by the measurement unit.