An infinite-layer structure artificial superlattice thin film, [CaFeO2]/[SrFeO2], was prepared by low-temperature reduction of a brownmillerite-structure artificial superlattice thin film [CaFeO2.5]/[SrFeO2.5] grown on a SrTiO3(001) substrate. Both the CaFeO2.5 and SrFeO2.5 brownmillerite structures changed to the infinite-layer structures and unusual square-planar-coordination FeO2 plane was stabilized in the infinite-layer superlattice. The low-temperature reduction technique is useful for making superlattices that cannot be made by simply depositing a film. (C) 2010 The Japan Society of Applied Physics