To stably feed the power by realizing the constant control of the electromotive force obtained by an MHD generator without relying on an inverter.

An MHD power generating system is provided with a storage means 21 in which the electric output characteristic curve of the quantity S of seeds and the electric output W of an MHD generator 4 and the output voltage characteristic curve of the seed quantity S and the output voltage V of the MHD generator 4 are stored, and a control means 20 for controlling the quantity S of seeds so that the electric output W of the MHD generator 4 becomes maximum based on the electric output characteristic curve and the output voltage characteristic curve, and the seed quantity S of seeds at which the electric output W of the MHD generator 4 becomes maximum is poured in the working fluid 1.