To prevent a key display part from wearing by a method wherein on the rear surface of a surface area, a primer treatment is performed, and an elastomer layer is formed by printing reverse characters, and a plastic film is filled in a female die which becomes a key top part in such a manner that the reverse characters may locate on the top, and the plastic film is integrally formed with an elastomer sheet body.

This key top body is formed of a printing device 20 for a plastic sheet 40 and a die device 30 for a silicone based elastomer body. Reverse characters are printed on the plastic sheet after a primer treatment, on a supporting body 41 by a printing block 23. For the die device 30, a female die 33 is formed on a base 31 in such a manner that the central position of the die device 30 may become common with the central position of a plate 24 of the printing device 20. A punching die which cuts the plastic sheet into a key shape, is set in the printing device, and the sheet 40 is punched. A key top is formed between the female die 33 and a male die, and the film of the sheet 40 which is filled in the female die 33, is integrally formed with a silicone based elastomer sheet by heating and pressurizing.