A photochromic display device using WO₃ thin film in contact with reducing agent was investigated. In this device, blue color was caused by patternwise illumination and could be erased by voltage application. It was found that the coloration rate was limited by diffusion of proton in the WO₃ film and its efficiency was proportional to the concentration of reducing agent. Slow bleaching (relaxation time was 5 hr) of the color was observed when the WO₃ film was left in the air, but the film in contact with reducing agent showed the memory effect.