A correction value determination unit determines a light quantity correction value for each main scanning position of each original based on a reading value obtained by an image reading unit reading a reference member. An anomaly detection unit detects, based on the light quantity correction value with respect to the preceding original and the light quantity correction value with respect to the succeeding original, an anomaly in the correction value, and specifies a main scanning position at which the anomaly occurs. A modification unit modifies a reading value at the main scanning position at which the anomaly occurs, out of reading values obtained from the reference member with respect to the succeeding original, or a correction value for the main scanning position at which the anomaly occurs, out of light quantity correction values with respect to the succeeding original.