Disclosed is a clay thin film substrate improved in surface planarity and gas barrier properties. Also disclosed is a display using such a clay thin film substrate. Specifically disclosed is a clay thin film substrate wherein a gas barrier inorganic layer (12) is arranged on at least one side of a clay thin film (11) in which clay particles are oriented and arranged in layers. Also disclosed is another clay thin film substrate wherein a planarization inorganic layer or an organic polymer layer is arranged on at least one side of a clay thin film in which clay particles are oriented and arranged in layers, and a gas barrier inorganic layer is arranged on the planarization inorganic layer or the organic polymer layer. The gas barrier inorganic layer is composed of a silicon oxide film containing nitrogen, and the planarization inorganic layer is composed of a silicon oxide film containing carbon. These clay thin film substrates can be suitably used as substrates for electroluminescent displays and liquid crystal displays.