PURPOSE: To prevent an error in thickness measurement from being generated even when a high-temperature body is measured by moving bars for compensation which is made of a body with a small coefficient of thermal expansion or high heat conductivity in the width direction of the body to be measured and measuring its variation by a sensor for compensation.
CONSTITUTION: If a temperature gradient is generated between the surfaces A and B of frames 6a and 6b like the measurement of a high temperature body to be measured, the surfaces A and B of the frames 6a and 6b differ in the extent of elongation and the frames 6a and 6b deflect. Displacement sensors 2a and 2b and processing parts 3a and 3b are affected by the deflection to vary in thickness output. The bars 10a and 10b for compensation are small in coefficient of thermal expansion and each fixed to a base 8 at only one end, so their elongation is small and a little deflection is caused. The outputs of the displacement sensors 11a and 11b for compensation show the measured deflection of the frames 6a and 6b. The processing parts 12a and 12b convert their deflection quantities into errors and a compensation processing part 17 compensates the thickness output of the processing part 3b to evade the generation of the thickness measure due to the temperature gradient.