A light receiving portion is configured to receive light that varies with time while a mark formed on an object moves across a detection area. The position of the mark on the object is determined based on comparison of a time-varying level of the light with at least one threshold during movement of the mark on the object across the detection area. The determined position of the mark is corrected by a correction value into a corrected mark position. The correction value is set to a higher value, if the time-varying light level exceeds the threshold or falls below the threshold with a smaller slope while the mark moves across the detection area. An image forming position is adjusted based on the corrected mark position.