PURPOSE: To save man-hours for measurement and to improve accuracy by providing a mechanism for judging the slant of a sample and a mechanism for adjusting the angle, and automatically adjusting the slant angle of the sample.

CONSTITUTION: A sample under measurement 1 is heated by a heater 2 in a vacuum container 14. The deformation of the sample 1 is observed by an interferometer 3. The image of the obtained interference fringes is processed in an image processor 6. Thereafter, the image is sent into a slant-angle analyzing part 11. In the analyzing part 11, the slant angle of the sample 1 is judged based on the interference fringes. The slant-angle correcting amount required for the sample 1 is obtained based on the judged result. Then, a computer 8 outputs an instruction to a motor driver 12 based on the correcting amount for obtaining the optimum slant angle. Thus, a motor 13 is driven, and the slant angel of a slant stage 10a is adjusted. After the adjustment, the slant angle of the sample 1 is judged again. The adjustment is repeated until the slant angle becomes the optimum angle for analyzing the radius of curvature. Then, the image of the interference fringes is sent into a curvature-radius analyzing part 7, and the radius of curvature of the sample is analyzed. Finally, the thermal expansion coefficient of a thin film is computer based on the relationship between temperature and the radius of curvature.