A polymer alloy comprising at least two thermoplastic-resin ingredients. It is useful for providing a structural material having excellent mechanical properties or a functional material having excellent regularity. It can have a finely controlled structure which has been evenly dispersed. Also provided is a process for producing the polymer alloy. The polymer alloy made of at least two thermoplastic-resin ingredients is produced by a process in which a precursor (A) for at least one of the thermoplastic-resin ingredients constituting the polymer alloy is chemically reacted in the presence of a remaining thermoplastic-resin ingredient (B1) and/or thermoplastic-resin ingredient precursor (B2) to induce spinodal decomposition. This polymer alloy hence has excellent regularity and can have a finely controlled structure which has been evenly dispersed.