An image forming apparatus which forms a color image on a print medium by overlapping an image formed in each color, the image forming apparatus comprising: a plurality of developing units each configured to develop a color image in a predetermined color; a transfer unit configured to transfer the color image developed by the plurality of developing units to the print medium, the transfer unit comprising a transfer belt rotatably installed on a transfer path; characterised by: a first detector which is provided in a predetermined location facing the transfer belt, the first detector being configured to detect any defect from a surface of the transfer belt; a second detector which is provided in a predetermined location of facing the transfer belt and spaced from the first detector, the plurality of developing units being located between the first detector and the second detector, the second detector being configured to detect a test pattern of an auto color registration error transferred to the transfer belt; and a controller configured to adjust data relating to the test pattern of the ACR error formed in a defective location of the transfer belt detected by the first detector either by causing the data on the test pattern of the ACR error formed in the defective location of the transfer belt detected by the first detector not to be reflected in an adjustment of the ACR error, or by controlling the developing units so as to form the test pattern of the ACR error in a location avoiding a defective location if the first detector detects the defect on the transfer belt. a plurality of developing units each configured to develop a color image in a predetermined color; a transfer unit configured to transfer the color image developed by the plurality of developing units to the print medium, the transfer unit comprising a transfer belt rotatably installed on a transfer path; characterised by: a first detector which is provided in a predetermined location facing the transfer belt, the first detector being configured to detect any defect from a surface of the transfer belt; a second detector which is provided in a predetermined location of facing the transfer belt and spaced from the first detector, the plurality of developing units being located between the first detector and the second detector, the second detector being configured to detect a test pattern of an auto color registration error transferred to the transfer belt; and a controller configured to adjust data relating to the test pattern of the ACR error formed in a defective location of the transfer belt detected by the first detector either by causing the data on the test pattern of the ACR error formed in the defective location of the transfer belt detected by the first detector not to be reflected in an adjustment of the ACR error, or by controlling the developing units so as to form the test pattern of the ACR error in a location avoiding a defective location if the first detector detects the defect on the transfer belt. a first detector which is provided in a predetermined location facing the transfer belt, the first detector being configured to detect any defect from a surface of the transfer belt; a second detector which is provided in a predetermined location of facing the transfer belt and spaced from the first detector, the plurality of developing units being located between the first detector and the second detector, the second detector being configured to detect a test pattern of an auto color registration error transferred to the transfer belt; and a controller configured to adjust data relating to the test pattern of the ACR error formed in a defective location of the transfer belt detected by the first detector either by causing the data on the test pattern of the ACR error formed in the defective location of the transfer belt detected by the first detector not to be reflected in an adjustment of the ACR error, or by controlling the developing units so as to form the test pattern of the ACR error in a location avoiding a defective location if the first detector detects the defect on the transfer belt.