Image formation apparatus capable of detecting and correcting positional offsets

By means of a plurality of image carrying units, a correction mark formation unit transfers mixed color mark trains in right and left lines onto a moving member in the form of a belt. The mixed color mark trains serve as marks for correcting image offsets between color images and each consist of K-C, K-M and K-Y two-color marks which are superposed on each other in an offset manner. A correction value calculation unit detects a lightness pattern of the mixed color mark trains transferred onto the moving member and figures out correction values for image offsets between color images from the phase of this lightness pattern. A positional offset correction unit automatically corrects the offsets between the color images on the basis of the correction values acquired by the correction value calculation unit.