PURPOSE: To obtain a fuel battery generating system which can supply electric power stably for a long time by improving utilization rate of a fuel battery by means of a steam flow controller which controls the adjusting valve of steam flow by feeding back the flow signal of the steam flow detector.

CONSTITUTION: An air flow signal F transmitted from an air flow detector 9; a temperature signal T transmitted form an intenal temperature detector 10 of the fuel battery; and load current signal I transmitted from a current detector 11; are all input in an arithmetic unit 12 which calculates the target signal W ref corresponding to the steam amount which should be applied to humidity the air supply line. Moreover, the steam flow regulator 13 controlls the steam flow control valve 8 so as to establish the target signal W ref by feeding back the steam amount signal W transmitted from the steam flow detector 7. The optimum concentration can be maintained as well as drying of phosphoric acid electrolyte can be prevented as the air is humidified to a specified flow rate, when the load is low with a small load currennt I, even through the phosphoric acid electrolyte concentration tends to rise.