In an image forming apparatus, an error detection mark is formed by using an image forming unit on the surface of a carrier along a detection line passing through an exposure position in a carrier moving direction. The error detection mark includes a first mark and a second mark spaced from each other by a fixed distance in the carrier moving direction. The error detection mark is measured by the sensor. A measurement error of the sensor for a registration mark is determined based on the measured values for the first mark and second mark. A registration mark is formed by using the image forming unit on the surface of the carrier on the detection line. The registration mark is measured using the sensor. Offset of an image-forming position is corrected based on the determined measurement error and a measured value for the registration mark.