PURPOSE: To accurately measure the temperature of an object even if the object fluctuates in state by a method wherein the change of the object in outer diameter is measured, and the temperature of the object is computed basing on the measured outer diameter.

CONSTITUTION: ITV cameras 7 and 8 pick up the image of the outer edge of a semiconductor wafer 3, and the image data are converted into binary data through an input 11, and stored as binary image data in an image processing device 12. An outer diameter change caused by thermal expansion in the semiconductor wafer 3 is obtained through the image processing device 12 basing on the binary image data. The outer diameter change of the semiconductor wafer 3 outputted from the image processing device 12 is inputted into a temperature computing device 13, and the temperature of the semiconductor wafer 3 is obtained basing on an outer diameter change in accordance with a temperature change. Therefore, even if a film formed on the rear side of the semiconductor wafer 3 is changed, the Si semiconductor wafer 3 is kept constant in linear thermal expansion, so that the temperature of the semiconductor wafer 3 is accurately measured basing on its outer diameter change.