The molecular composite material having excellent properties such as mechanical strength, heat resistance, solvent resistance, etc. is produced by (a) polymerizing an aromatic diaminodithiol compound, of which hydrogen atoms of thiol groups of aromatic diaminodithiol compound are substituted with substituted or unsubstituted alkyl groups, with a dicarboxylic acid derivative to form an aromatic polythiazole prepolymer, (b) dissolving the aromatic polythiazole prepolymer in an organic solvent to prepare a solution thereof; (c) adding a monomer of the matrix polymer to the solution to polymerize the monomer to the matrix polymer in the solution; (d) removing the organic solvent from a mixture of the aromatic polythiazole prepolymer and the matrix polymer; and (e) heating a mixture of the aromatic polythiazole prepolymer and the matrix polymer to cause a thiazole ring closure reaction of the aromatic polythiazole prepolymer.