There are provided an electrode substrate for electrolysis comprising an electrically conductive metallic base material and a coating layer having a thickness of from 10 to 200 .mu.m composed of a platinum group metal and partial oxides of a non-stoichiometric composition containing at least one kind of metal selected from the group consisting of titanium, tantalum and niobium, and oxygen formed on the surface of the base material and a method of producing the electrode substrate. The electrode substrate has a sufficient durability and a chemical stability under a high electric current density, and can be stably used for a long period of time for use under a cathodic polarization in an electrolysis, etc., with a positive-negative inversion, and the use in an electrolyte containing corrosive materials.