PURPOSE: To improve the accuracy and reproducibility by providing a detection rod with a cooler and holding the detection rod thermally equivalent to a detection tube.

CONSTITUTION: A sample 1 which is heated by a heater 2 is fixed with a cooling tube 4 and an extrusion rod 5 which is nearly as long as the cooling tube 4 and also has a nearly equal coefficient of heat expansion, the displacement of a core 8 pertaining to the thermal expansion of the sample 7 is measured by a differential transformer 7, and the temperature of the sample 1 by the heater 2 is measured by a thermostat 3 to calculate the coefficient of thermal expansion. This rod 5 is provided with the cooling plate 12 of the cooler across a pedestal 13 which has excellent heat conductivity and the cooling tube 4 which is cooled through a reference plate 6 and the rod 5 becomes thermally equivalent. Consequently, the displacement of the tube 4 and the displacement of the rod 5 which are both caused with temperature cancel each other to exert no influence upon measurement, thereby improving the accuracy and reproducibility when the coefficient of thermal expansion is measured.