A pusher formed of a rubber material, a click plate, a membrane switch which serves as a second step switch, a reinforcing plate, a membrane sheet on which a plurality of first step switches are formed, a key operation base and a knob are sequentially assembled into a case. The knob includes a plurality of depressing pieces having on its internal surface a projection which moves into contact with each membrane switch and also includes a marginal portion which is supported by the top surface of the case. The knob is thin-walled except for the depressing piece, and the thin-walled portion comprises a thermoplastic elastomer or silicone rubber. The knob, the key operation base and the reinforcing plate are secured together. As the depressing piece is depressed, the thin-walled portion which is located therearound becomes flexed, whereby the membrane switch is depressed by the projection to turn the switch on. When the depressing piece is further depressed, the reinforcing plate moves and the click plate reverses, turning the switch on.