The object of this invention is to provide a transparent plastic substrate having better surface smoothness than prior substrates, as well as having high transparency and high gas-barrier characteristics, and to provide a flexible display element using that uses this substrate. For that purpose, a gas-barrier transparent plastic substrate is obtained such that a transparent oxide film comprising either of a tin-oxide amorphous film, or tin-oxide amorphous film containing at least one added element that is selected from among the group of silicon, germanium, aluminum, cerium and indium, at a ratio of 0.2 to 45 atomic % with respect to the total of added element and tin, is formed as gas-barrier layer on at least one surface of a plastic film base material. It is possible that the gas-barrier transparent plastic substrate be formed as bilayer where a silicon-oxide film or silicon-oxynitride film is formed on the transparent oxide film. When a transparent electrode film is further formed on it, a flexible display element can be obtained.