Abstract: Substantially triangular shaped register marks are applied to respective plate cylinders of a multicolor printing machine. Each register mark is made up of a first side extending laterally of axial direction of a plate cylinder and a second inclined side extending downwardly to an end of the first side. Rotation of register marks is detected by a photoelectric detector and a rotary encoder rotating in synchronism with respective plate cylinders generates a reference pulse and a rotation pulse having a shorter period than the reference pulse. Based on the reference pulse, the rotation pulse and an output of the photoelectric detector, a leading edge time difference signal and a trailing edge time difference signal are formed and then integrated by integrators. In response to outputs of the integrators the peripheral direction and the lateral direction of the respective plate cylinders are adjusted.