PURPOSE: To reduce the size and to lengthen the service life by arranging a gas turbine generator in the downstream of a rare gas compressor.

CONSTITUTION: In a combined generating system, high temperature rare gas E1 heated by a heat source D is fed to an MHD generating unit A1 in order to perform MDH power generation, then the discharged rare gas E2 is cooled through a heat exchanger C1 and fed to a compressor B thereafter compressed rare gas E6 is fed to a gas turbine generator A3 in order to generate power. Rare gas E5 discharged from the gas turbine generator A3 is returned to the heat source D thus constituting a closed loop. Since the pressure of the rare gas E6 to be fed to the gas turbine generator A3 can be set higher, through the compressor B, than that of the rare gas E1 to be fed to the MHD generating unit A1, the size of the gas turbine generator A3 and the rare gas compressor B can be reduced.