A separator for a fuel cell allows air to bypass a diffusion part, which is frequently exposed to air, and thus flow directly to a reaction surface, which can reduce deterioration of a polymer electrolyte membrane. The separator includes a separator main body having a diffusion part formed thereon that is configured to allow air to be diffused and supplied from an air inlet manifold to the reaction surface; and a gasket line formed on the separator main body and surrounding the air inlet manifold and the reaction surface to maintain airtightness. The separator main body or the gasket line includes a bypass flow path formed thereon so as to allow air supplied from the air inlet manifold to flow directly to the reaction surface without passing through the diffusion part.