TiO2-SnO2 composite with the modulated lamellar structure was obtained within a short period by microwave irradiation using 28-GHz frequency. Single-phase solid solution with the rutile-type structure was formed at the early stage of irradiation, and the prolonged irradiation caused the formation of mesoscopic lamellar structure induced by spinodal decomposition. Such a nonequilibrium nature is believed to be due to selective heating behavior under microwave irradiation.