This invention is a method for manufacturing a polymer alloy, by making at least two resins used as components miscible, and inducing the spinodal decomposition for causing phase separation, for forming a co-continuous structure with a wavelength of concentration fluctuation of 0.001 to 1 .mu.m or a dispersed structure with a distance between particles of 0.001 to 1 .mu.m. This invention is also a polymer alloy manufactured by the method. The polymer alloy of this invention can provide a molded article, film, fibers and the like respectively with excellent mechanical properties at high productivity.