To freely process a seal surface in a short time without using a character matrix by previously impregnating a seal material made of foamable synthetic resin with printing ink, and applying a laser beam to its surface.

A foamable synthetic resin is used as a seal material 21, cut in a predetermined size, and dipped in a printing ink container and impregnated with the ink. Thereafter, the material 21 is mounted on a table 27 of a processing machine. The table 27 is vertically moved to match a laser spot 1a to a surface of the material 21. Subsequently, a blower 26 is driven to suck the air in a sealed space 22. A compressed air source 18 is driven to inject compressed air from an injection nozzle 10. After the above regulation, a laser beam 1 is emitted to a processing surface, and processing of the seal surfaces is started. Then, a part emitted with the beam is melted, but cooled with ink impregnated with the material 21 and only the surface of the material 21 is melted. Thus, pores of the surface are collapsed to form a closed surface.