Polymeric solid electrolyte with high ionic conductivity of about 10-⁵S/cm is prepared to construct all solid state WO₃-based electrochromic display device (ECD). Poly [oligo (oxyethylene) methacrylate]/LiClO₄ hybrid thin film is cast on both ITO glass electrode and WO₃ coated ITO glass electrode. These are contacted with the polymeric solid electrolyte film-coated sides under dry argon atmosphere. The increase in the applied voltage resulted in the increase of the differential optical density change between coloring and bleaching processes for this ECD. The optical density at 800 nm for the coloring of WO₃ is proportional to the injected charges. The coloring state of this ECD has been maintained for several weeks under open circuit, deeply depending on the anion migration in polymeric solid. electrolyte. From the time dependence of the current passing through the cell in coloring and bleaching processes, the coloring and bleaching mechanisms for this ECD are interpreted to be the same as those for ordinary ECD composed of electrolyte solution system. Cycling of coloring and bleaching is carried out for more than several hundred times without any decay in the activity and efficiency.