A fuel cell includes a separator. A constant amount of air is supplied to the fuel cell irrespective of positions within an air channel, and thus, degradation of the fuel cell is prevented. The separator includes a separator body and a porous structure which has a plurality of pores defined therein to provide a path through which a fluid flows, where the separator body includes: a fluid inlet part having a space into which the fluid is introduced; a reaction region configured to receive the fluid; and a diffusion part which is provided between the fluid inlet part and the reaction region, where the porous structure is stacked on one surface of the reaction region, and the number of pores per unit volume of the porous structure varies in an inlet region.