An automatic aspect ratio discrimination apparatus for detecting a picture starting position, a picture ending position and a subtitle ending position by generating three histograms segmented by two slice levels S and T at every line from a luminance signal Y at a comparison circuit and counters, judging when the line is black, a picture or a subtitle by inputting the obtained histograms to the comparison circuit, detecting lines varying from black to picture, from picture to black, and from subtitle to black at an edge detection circuit and judging if the lines varying from black to picture and from picture to black are stable in time or not at a temporal filter.