The present invention provides a base substrate for epitaxial diamond film capable of epitaxially growing a large area of high quality diamond, having a diameter of 1 inch (2.5 cm) or more, on an iridium base by using the CVD method, a method for producing the base substrate for epitaxial diamond film, an epitaxial diamond film produced with the base substrate for epitaxial diamond film and a method for producing the epitaxial diamond film. An iridium (Ir) film is formed by epitaxial growth on a single crystal magnesium oxide (MgO) substrate or a single crystal sapphire (α-Al2O3) substrate by means of a vacuum deposition method or a sputtering method, and a bias nucleus generation process of forming epitaxial diamond nuclei is applied to the surface of the iridium (Ir) base formed as a film by exposing an ion-containing direct current plasma to the surface of the iridium (Ir) base formed as a film.