PURPOSE: To make the sampling rate of enthalpy constant by regulating the distance between sidewalls for a working fluid to pass through at the time of variation of the flow of the working fluid through variation of an electric power demand.

CONSTITUTION: A variable passage width disc type MHD generator is composed of disc sidewalls 1, 2. The inflow working fluid 3 of the generator flows from a nozzle part 4 provided in the center of one sidewall 1 into MHD generator, is accelerated in a throat part 5 provided between the sidewall and the other sidewall 2 and then flows radially, passes through a generator part between an annular anode 6 provided on the small diameter side and an annular cathode 7 provided on the large diameter side, and passes through a diffuser part 8 and as an outflow working fluid 9 outward of the generator. Further, because the flow rate of the working fluid 3 is required to increase at the time of increase of an external load, the sidewalls 1, 2 are moved to vary the distance 10 so that it is proportioned to the increase. Thus, it is possible to regulate the cross section of a passage including the throat part 5.