An optical filter made of inorganic material for blue light which transfers blue light and absorbs all of the other colored lights can be realized by sticking an inorganic mixture powder pasted by mixing with screen oil on a glass substrate and firing it, which mixture includes transparent glass material powder and a combination of oxides of cobalt and aluminum. The contrast of picture displayed on a display device can be extremely increased by reducing the reflection of incident ambient light without the reduction of brightness on account of the application of the above optical filter onto the displaying screen.