An image forming apparatus includes: a rotatable belt member; a toner image forming unit which forms a toner image on the belt member; a first detection unit which detects the density of the toner image transferred to the belt member; a plurality of position marks which detects a position in the circumferential direction of the belt member; a second detection unit which detects the position marks; a calculation unit which calculates the position in the circumferential direction of the belt member based on a detection result of the second detection unit; and an adjustment unit which adjusts toner image forming conditions of the toner image forming unit based on the output of the first detection unit and the circumferential direction position calculated by the calculation unit; wherein the shape of each of the plurality of position marks is different at each circumferential direction position.