1. An image forming apparatus comprising: an image bearer; an image forming device including a developing device to develop, with a developer including a toner and a carrier, a latent image on a surface of the image bearer into a detection toner image, the developing device including a developer bearer disposed facing the image bearer, the developer bearer to bear the developer; a toner concentration detector to detect a toner concentration of the developer in the developing device; a toner supply device to supply the toner to the developing device; an image density detector disposed facing the surface of the image bearer to detect an image density of the detection toner image on the surface of the image bearer; and a controller including a memory device to store a target concentration for the toner concentration and a target density for the image density of the detection toner image, wherein, at a predetermined adjustment timing before image formation is started, the controller causes the image forming device to form the detection toner image on the surface of the image bearer, adjusts a developing potential, which is a difference between a developing bias applied to the developer bearer and a latent image potential on the image bearer, to keep the image density of the detection toner image detected by the image density detector at the target density, and drives at least one of the toner supply device and the developing device to keep the toner concentration detected by the toner concentration detector at the target concentration, wherein, after the image formation is started, the controller drives the toner supply device to keep the detected toner concentration at the target concentration, wherein, at a predetermined sheet interval timing after the image formation is started, the controller causes the image forming device to form the detection toner image in a non-image area on the surface of the image bearer, adjusts the target concentration based on the detected image density of the detection toner image, and controls the toner concentration based on the adjusted target concentration, and wherein, when the detected toner concentration is higher than the target concentration plus a threshold, the controller suspends the image formation, causes the image forming device to form the detection toner image on the surface of the image bearer, and readjusts the developing potential to keep the detected image density of the detection toner image at the target density.