A printing ink is vulcanized on the top surface of an operation indicating member consisting of a silicone-type elastomer and is integrated with the elastomer to form a character of a predetermined shape on the top surface. The printing ink consists essentially of a material which exhibits a vulcanizing mechanism similar to that of the silicon-type elastomer. In manufacturing the operation indicating member, the predetermined character is formed by means of the printing ink on a transfer plate disposed within a mold for molding the body of the indicating member. Next, the silicon-type elastomer is introduced into the mold and heated under elevated pressure to vulcanize and integrate the character on the transfer plate and the silicone-type elastomer, whereby the character is integrally embedded in the top surface of the operation indicating member formed in the mold.