A semiconductor device has a first and a second semiconductor chip (14, 16). On the surface of the first semiconductor chip (14), a plurality of first electrodes are formed. On the surface of the second semiconductor chip (16), a plurality of second electrodes are formed. By placing the first and the second semiconductor chip (14, 16) so that their surfaces may face each other, the first electrodes and the second electrodes are connected to each other. On the surface of each of the semiconductor chips (14, 16), circuit elements are also formed and these circuit elements mounted on the first and the second semiconductor chip (14, 16) are covered by the second and the first semiconductor chip (16, 14) respectively. The connections of the first and the second semiconductor chip (14, 16) are sealed in a package (26) by synthetic resin which has an excellent moistureproof property, and the entire bodies of the first and the second semiconductor chip (14, 16) are airtightly sealed in a package (22) by the second synthetic resin.