A key input device comprises a dome sheet 7 provided over a base plate 5 and having a plurality of dome portions, and a plurality of manual keys disposed above the dome sheet 7 for depressing the respective dome portions. Each of the dome portions is elastically restorably inverted by being depressed by the corresponding manual key to close contacts. The dome sheet 7 has a flat sheet portion 70 formed around the dome portions and comprising an inner region 74 extending inwardly of dome portions 72 to be selectively depressed by a seesaw key 15, and an outer region 73 extending outwardly of the dome portion 72. A spacer sheet 6 is interposed between the base plate 5 and the dome sheet 7 to thereby position the inner region 74 of the flat sheet portion 70 of the dome sheet 7 at a lower level than the outer region 73. Accordingly, key contacts can be reliably closed by the seesaw key 15.