PURPOSE: To accurately measure the temperature of a substrate without being influenced by the hysteresis of a semiconductor wafer by a method wherein a crystal face gap of a substance at a desired measurement point of a semiconductor device substrate or of a film spreading the substrate is measured by utilizing the X-ray diffraction method.

CONSTITUTION: A crystal face gap of a substance at a desired measurement point of a semiconductor wafer 2 substrate or of a film spreading the substrate is the function of only temperature if the substance is determined. For example, a wafer 2 is arranged in the chamber 1 of a RTP(rapid thermal process) equipment, and an X ray is entered into a desired measurement point of its rear to control the position of an X-ray detector 6 by a controller 11 so as to be adjusted at a specified crystal face diffraction wave peak position which depends upon the X-ray diffraction angle θof a crystal face of the substance of a measurement point (X-ray diffraction position). On the basis of positional information (clear from the controller 11) of the detector 6, the X-ray diffraction angle θ is determined by an operator 12 to calculate a specific crystal face gap of a measured substance; converting it into temperature enables the measurement of wafer temperature.