The object of the invention is to provide a display device of which the display is visible in a dark place or under strong external light. The present invention is a display device which performs display, changing the grayscale number corresponding to the external light strength, and a display device which can switch the display mode corresponding to contents displayed on the screen. The display contents include a text display mode displaying mainly characters and symbols, a picture display mode displaying images with a small number of colors such as a comic, a video mode displaying natural images with a large number of colors such as a photograph and a moving image, and the like. By switching the grayscale number arbitrarily according to these display modes, visibility can be ensured in a wide range from a dark place or under an indoor fluorescent light to under outdoor sunlight. For example, the grayscale number is switched so that display of from 2 to 8 grayscales is performed in the text display mode, display of from 4 to 16 grayscales is performed in the picture display mode, and display of from 64 to 1024 grayscales is performed in the video mode.