A resin substrate of the present invention has a resin layer and a surface layer formed on a surface of the resin layer, wherein the surface layer is a layer comprising silicon nitride as a main component and deposited by the chemical vapor deposition method, and at the interface between the resin layer and the surface layer, at the interface between the resin layer and the surface layer, an interfacial region over which a percentage changes from 80% to 20% has a thickness of not more than 25 nm, wherein the difference between the maximum nitrogen concentration in the surface layer and the steady-state nitrogen concentration in the resin layer is taken as 100%. The surface layer has an average surface roughness (Ra) of not more than 1 nm. The resin substrate has properties of water vapor barrier and surface flatness.