A key switch device comprising a mounting member made separate from a support plate or backing board. The key switch device comprises first and second link members are cross-linked with each other to act like scissors; the key top having receiver portions formed in an underside thereof for movably or pivotably receiving upper shaft pairs each formed at upper ends of each of the first and second link members; a support plate disposed under the key top and having a plurality of through holes in predetermined positions; a membrane member disposed on the support plate under the key top, the membrane member having a contact point for contacting an elastic switch as the elastic switch is pressed down by the key top and openings formed around the through holes in the support plate to expose the through holes; and a mounting member separate from the support plate, the mounting member including fixing portions for movably or pivotably fixing lower shaft pairs each formed at lower ends of the each link member and fastening portions fixedly fastened with an underside of the support plate via the through holes in the support plate. The invention imparts a simple structure to the support plate.