PURPOSE: To enable an ink to be prevented from spreading in a plane direction as well as to penetrate deep by a method wherein, after a primary heat treatment is conducted as a sublimating process for penetrating the ink into a plastic, a secondary heat treatment is carried out after a remaining ink on the plastic printing surface is removed.

CONSTITUTION: An ink consisting of a sublimating dye is printed on a key to surface of a keyboard and air-dried. Thereafter, as a sublimating process, a primary heat treatment is conducted under a temperature of 130°C for 5min, and a remaining ink on the printing surface is removed by washing. Moreover, a secondary heat treatment is carried out under a temperature of 160°C for 10min. In the primary heat treatment, the temperature and the time are set lower and shorter than in the secondary heat treatment. This manner enables the good prevention of spreading of the ink in a plane direction. The secondary high-temperature and long-time heat treatment enables the ink to penetrate deep.