A solid catalyst having a close-packed structure has basic structural units present in the surface of the solid catalyst, the basic structural units including (i) a triangular lattice constituted of atoms of platinum, ruthenium, and at least one additional element which are disposed at the vertexes in the triangular lattice so that each atom of one of the elements adjoins atoms of the other elements or (ii) a rhombic lattice constituted of atoms of platinum, ruthenium, and at least one additional element which are disposed at the vertexes in the rhombic lattice in an atomic ratio of 1:2:1 so that each ruthenium atom directly adjoins a platinum atom and an atom of the additional element; and a fuel cell includes either of the solid catalyst as an anode-side electrode catalyst.