PURPOSE: To enable an amount of a seed agent to be injected from a seed- injecting device to an inlet pipe of an MHD generator and an amount of supply from a seed-supply device to the seed-injecting device to be controlled efficiently and at the same time the inlet pipe of the high-potential MHD generator to be insulated from the ground-potential seed-supply device positively.

CONSTITUTION: A plurality of seed agent storage tanks 16a and 16b are provided on a injection pipe 15 of a seed agent which gives conductivity to a rare gas as a working fluid of a closed cycle MHD generation system and these storage tanks are circulated alternately or in sequence. Also, electrically insulated cutoff valves 18a, 18b, 19a, and 19b are provided at an entrance and an exit of the seed agent storage tank.