To firmly stick a potting layer by forming the potting layer made of transparent resin on a button top face of a key mat type plastic film in which a pattern layer is formed.

A plastic film 2, in which a pattern layer 4 is formed on one side face or both side faces, is formed into a key mat shape. On the formed button top face, a potting layer 3 is arranged by applying a potting process to transparent resin. In addition, the potting layer 3 in the plastic film 2 is arranged in a recess part in a metal die matching with the button shape so as to face the recess part, and then an uncured silicone rubber composition is arranged on them. The silicone rubber is cured by clamping the metal die. As the plastic film 2, a flexible extensible plastic film provided with deep drawability by molding is preferable. Molding of the plastic film 2 into a key mat shape is carried out through vacuum molding method and the like.