A register error amount detection apparatus obtains the register error amounts between the colors of patterns printed on a printing product based on the positions of register marks in a captured image of the printing product. The register marks have different colors including a reference color and are printed in a line at intervals a in the vertical direction. The register marks all have the reference color and are printed in a line at the intervals a in the vertical direction at positions translated from the positions of the register marks by a distance b in the horizontal direction.