NOVELTY - An opening adjustment unit adjusts the opening of a gas inlet and a gas outlet depending upon the variation in the characteristics of the heating gas. Consequently, the flow rate of the heating gas through the gas inlet and outlet are adjusted. DETAILED DESCRIPTION - A supply unit supplies heating gas to the surface of gas diffusion electrodes (16,17) through a gas supply path. An emission unit ejects the heating gas from the gas diffusion electrodes. A gas inlet that opens along the electrode surface guides the heating gas from the supply unit to the gas supply path. A gas outlet that opens along the electrode surface guides the gas from the gas supply path to the emission unit. A flow rate adjustment unit (40) has a humidity sensitive material (41) that expands depending upon the humidity of the heating gas. The humidity sensitive material supports a gas shielding material (42) that can be moved along the inclined surface (12b) of a separator (12), when the humidity sensitive material expands. As a result the clearance between the covered portion (44) of the shielding material and the bottom surface (12a) of the separator and the opening width of the emission path (26a) vary, so as to change the rate of flow of heating gas.    USE -   For adjusting rate of flow of heating gas in fuel battery used in power plants.    ADVANTAGE -   Avoids faults such as damage of electrolyte caused by continuous running of fuel battery in unsteady state. The composition is simplified and cost is reduced as electric equipments such as sensor and actuator are not required for adjusting the flow rate of heating gas. DESCRIPTION OF DRAWING(S) - The figure illustrates the composition of the heating gas flow rate adjusting mechanism for fuel battery. (12) Separator; (12a) Bottom surface; (12b) Inclined surface; (16,17) Gas diffusion electrodes; (26a) Emission path; (40) Flow rate adjustments unit; (41) Humidity sensitive material; (42) Gas shielding material; (44) Covered portion.