Prior to the formation of registration pattern images (Step S7), base information is acquired by means of a test pattern sensor detecting a surface of an intermediate transfer belt or particularly a surface of a pattern formation region thereof. A surface condition of the pattern formation region is acquired based on the base information thus acquired, while the formation of the registration pattern images is controlled based on the surface condition. In this manner, the registration pattern images are formed with an adequate consideration given to the surface condition of the pattern formation region where the registration pattern images are to be formed (Step S7). Hence, the positions of the registration pattern images may be detected with high accuracies. As a result, a proper correction of color registration offset may be accomplished, assuredly preventing the occurrence of the color registration offset or degraded color tone.