The morphology and layered structure of an alkyl ammonium bromide ultrathin film prepared on an aqueous solution of lead bromide were investigated on the basis of surface pressure-area curve measurements, reflection spectrum measurements, X-ray reflectivity measurements and atomic force microscopic observations. It became apparent that the layered structure sandwiched a two-dimensional inorganic layer between two organic monolayers was formed by folding the monolayer by compression on an aqueous PbBr2 solution.