PURPOSE: To reduce a number of storage heat exchangers or additional equipments to be operated by switching, to cut down an installation space and to reduce cost.

CONSTITUTION: During (A) the blow in order that gas to be heated is supplied to an MHD generator by one storage heat exchanger, the other storage exchanger is heated (B) by heated gas, the storage heat exchanger is evacuated after the heating, flushing of soot, etc., adhering to the storage heat exchangers is made by gas to be heated, and gas to be heated is filled (D). Before the blow of gas to be heated is completed by one storage heat exchanger, switching is made to start the blow in order that gas to be heated is supplied to the MHD generator from the other storage heat exchanger.