PURPOSE: To continuously obtain the value of temperature-compensated thicknesses by beforehand obtaining the characteristic curves of voltage and thicknesses and the characteristic curves of temperatures and thicknesses by experiments and beforehand programming sequence in a microcomputer.

CONSTITUTION: A high frequency energizing device 3 is provided to the surface of the sheet 1 of nonmagnetic to be measured at a spacing (d) and an electric field is generated in the sheet 1 by a high frequency electric power source 7 and a high frequency coil, whereby eddy current Ie is generated. In addition, the eddy current is detected by providing a detector 9 near the bottom surface of the sheet 1. A coil 11 and voltmeter 13 are provided to the detector 9. The induction voltage measured by the voltmeter 13 is corrected by the temperature of the sheet 1 obtainable by a thermometer 15, whereby the thickness of the sheet is obtained. At this time, the characteristic curve of the effect of temeratures upon the value of the thicknesses affecting the induction voltage is beforehand obtained and determined.