While a conveying error of a roller depends on the eccentricity of the roller, the amount and the state of the eccentricity sometimes makes the roller have different conveying errors from one point in the longitudinal direction of the roller to another. There is provided a construction for obtaining a correction value that is suitable for the correction of the conveying error even in such a case as described above. To this end, plural test patterns are formed in the longitudinal direction of the roller. Then, a suitable correction value for correcting the conveying error that depends on the eccentricity of the roller is obtained on the basis of these test patterns.