The present invention provides an automatic three-dimensional (3D) display device comprising: a display panel comprising arrays provided with basic pixels of four basic colors, and a columnar mirror device disposed above the display panel. The columnar mirror device array comprises a plurality of columnar mirror members extending in parallel to an inclined line, and an inclination angle of the inclined line relative to the rows of th pixels is tan-1 (a /mb), wherein, m represents the number viewpoints at adjacent lines before the inclination line appears, and a and b represent the vertical and horizontal length of each pixel. When n is 0 or a natural number, the number of viewpoints is 2m (2n +1), pixels corresponding to the given viewpoints repeat every m lines in each the columnar mirror member, the number of parallel lines extending in parallel to the inclined line through the pixels in the columnar mirror member is equal to the number of the viewpoints, and each of the parallel lines depends on the repeated basic pixels.