PURPOSE: To equalize the temperature of gas flowing into a duct, installed in the downstream of rare gas discharged from an MHD generator, easily to the temperature of normal exhaust gas regardless of temperature variation in the rate gas.

CONSTITUTION: A rare gas fed from a rare gas storage tank 3 is heated through a high temperature heat-exchanger 1 and introduced to an MHD generator 7 to generate power. The used rare gas is returned through a duct 9 and a return pipe 16 to the high temperature heat-exchanger 1. In such closed loop MHD power generation system, piping 21 for blowing a cooling rare gas from the rare gas storage tank 3 into the duct 9 is coupled to the joint of the MHD generator 7 and the duct 9.