On a sheet of paper on which an image has already been formed, a first registration mark pair and a second registration mark pair are printed. The first registration mark pair and second registration mark pair each consist of a pair of registration marks that are arranged to be spaced apart in a paper-width direction, the second registration mark pair located posterior to the first registration mark pair in the paper-conveying direction. Based on the actual positional relationship between the first registration mark pair and the second registration mark pair that are read by a reading unit, a correction unit corrects discharge data that corresponds to an image formation area located between the first registration mark pair and the second registration mark pair.