An example separator includes: a flat plate-shaped first plate member; a flat plate-shaped second plate member joined with the first plate member; an oxidation gas flow channel wall, which forms a flow channel of oxidation gas; a fuel gas flow channel wall, which forms a flow channel of fuel gas; a cooling medium flow channel wall, which forms a flow channel of a cooling medium; a first through hole, which penetrates the first plate member and the second plate member; a second through hole, which penetrates the first plate member and the second plate member; a first cooling medium passage part; a second cooling medium passage part; one projection, which is formed on at least one of the first cooling medium passage part and the second cooling medium passage part; and another projection, which is formed at a position corresponding to the one projection.