To increase dielectric strength and reduce insulation size in a package by selecting a gas having a better insulation property than a nitrogen gas which has been used in the conventional method, for a gas to be enclosed in the package of a flat semiconductor device for power.

In this flat semiconductor device, an IGBT chip 2 and a diode chip 3 are assembled in combination with collector electrodes 7, insulation partition walls 8 and the like in the sealed flat package 1 comprising an outer case (ceramic) 4 and case electrodes 5, 6; and then an insulation is enclosed in the package. For the insulation gas to be enclosed in the package, a carbon dioxide which has a better insulation property than nitrogen and whose leakage gas has no adverse effect on the surrounding areas is used instead of a nitrogen gas which has been used in the conventional method.