A conductor pattern surface of a plastically deformable substrate of a key switch device is insulated with a resist, and a switching circuit pattern is printed on the resist layer by using conductive ink. Carbon conductor ink is applied by printing on connectors and fixed contact patterns, and a mask is printed on the other portion. Mounting portions for mounting a guide supporting member are formed on the substrate by pressing the substrate from the lower surface thereof. An object of the invention is to integrate an electrical circuit pattern with a mounting portion of actuator structure so as to provide a key switch device which can reduce the thickness and the number of parts of a keyboard for a word processor, personal computer or any other device having a keyboard or keypad.