The invention relates to an improvement of the gas sealing at the periphery of the electrode base and separator which constitute the fuel cell. Provided herein is a method for sealing the gas of a phosphoric acid-type fuel cell which comprises filling the pores in the porous material periphery of the cell constituents with fine particles of at least one of silicon carbide and silicon nitride which have undergone surface oxidation treatment, and causing particles to expand in volume through the reaction of the surface oxide of the particles with hot phosphoric acid that yields phosphate compounds, thereby clogging the pores.