A fuel cell includes a membrane electrode assembly and first and second separators for sandwiching the membrane electrode assembly. A serpentine oxygen-containing gas flow field is formed on a surface of the first separator. A seal member is provided around the oxygen-containing gas flow field, i.e., around the anode. Filling seals are provided in a clearance between the seal member and the cathode at positions where the leakage of an oxygen-containing gas is likely to occur.