PURPOSE: To automatically set an initial register and an initial gate position at the replacement of a plate cylinder.

CONSTITUTION: A detection head 7 is provided for every plate cylinder 2. Using an encoder 9, a phase difference between corresponding plate cylinders is found. A phase correction angle is found for ensuring that register marks are printed at predetermined intervals. A phase correction is performed by phase correction means 14, 15. In addition, a gate position signal setting timing is arithmetically operated from the output data of the detection head 7 of the corresponding plate cylinder 2 and the encoder and positional data of the downstream detection head 7 and a downstream scanning head 8.