PURPOSE: To accurately print a discrimination mark to a folded paper box at the specific position thereof within a short time and to realize the alteration corresponding to the kind of the discrimination mark within the shortest time by using a laser printing apparatus to a part of a feed line.

CONSTITUTION: For example, the end surface 16b of a paper box 16 fed at constant high speed is detected by the detection device 18 of a photoelectric tube sensor and a laser beam oscillation command is outputted to a laser printing apparatus 10. A mask 24 having various marks 22 formed thereto by punching is mounted on a mask holder 20 and this mask holder 20 is inserted in a head part 12 and the laser beam generated from a laser oscillator is reflected by the mirror of the head part 10 to pass through the parts of the marks punched in the mask 24 and further passed through a condensing lens 26 to be focused to the printing position of the paper box 16. By this method, the color layer bonded to the surface of the paper box can be instantaneously incinerated and removed and various marks can be altered within a short time to be easily printed on the paper box.