PURPOSE: To detect whether a printing surface is right or not by a method wherein light is applied on a printing surface from a light emitting device to be reflected while the printing surface is being moved, and an integration value of an electric signal having been converted from a light accepting amount of each photodetector is compared with a reference value based on a number of dots distributed in a measuring track on a printing plate.

CONSTITUTION: A variation of a light accepting amount corresponding to a printing content is converted to an electric signal with each photodetector 18 following motion of a printing surface A, and enters an integrator E via an amplifier D to be integrated. Then, data is read out by inserting a magnetic card of a dot number on each track of a printing plate into a card reader X, and is inputted to a memory F through a corrector Y. Then, when respective electric signals corresponding to the memory F and a memory C are taken out to be inputted to an operation circuit H, the operation circuit H takes out an absolute value of a difference of respective electric signals as a difference in voltage, which is compared with a comparator J based on a setting value from an allowance setting device, and whether it is good or not is judged.