Abstract: An apparatus for automatic measuring thermal dimensional change from the room temperature to 2,000.degree. C., comprising, (a) means for supporting a specimen of the material to be tested and heating said specimen, (b) means for measuring the dimensional change of the specimen during heating said specimen, said measuring means having photo diode linear array and the telescopic lens for forming an image of the heated specimen, (c) means for illuminating the specimen from the opposite side of the photo diode linear array, said means having electric lamp and condensor lens, the radiation characteristic curve of said lamp being different from that of heated specimen, (d) means for separating the light forming image from the light radiated from said heated specimen, said means having light filter and neutral density filter between said specimen and said optical lens, (e) means for processing and calculating the video signal from the photo diode linear array indicating the results of measurements.