Samples of A1-0.1wt%Cu were solidified unidirectionally. Metallographic observations of the samples quenched during solidification showed that the stability-instability transition of the planar solid-liquid interface occured at the condition predicted by the perturbation theory, where the effect of interfacial energy on it was small. But the interfacial wave of the solid-liquid interface did not have the frequency predicted by the perturbation theory and was discussed.