PURPOSE: To increase the utilization rate of a fuel used in a fuel cell, stabilize the characteristic of the electrode, and lengthen the life of the cell by controlling the amount of steam mixed into an air supply line and a fuel supply line so that the concentration of electrolyte becomes constant.

CONSTITUTION: A mixer 5a works for giving steam to air sent from an introduction part (X), and a mixer 5b works for giving steam to a fuel gas sent from an introduction part (Y). According to the signals sent from a densitometer 6 which detects the concentration of phosphoric-acid electrolyte contained in a fuel cell 2, a controller 7 controls flow-controlling valves 40a and 40b, and controls the amount of steam contained in the air and the fuel gas. The air introduced from the introduction part (X), after being given steam by means of the mixer 5a, is supplied into the phosphoric-acid-type fuel cell 1. Here, the amount of steam mixed into the air is controlled with the flow-controlling valve 40a.