PURPOSE: To grasp accurately a temperature of a wafer, by providing a detector for detecting radiant light from the surface to be detected, a mask having many light transmitting openings and a lens means for forming an image of the surface to be detected on the surface of the mask.

CONSTITUTION: A mask 7 bored many minute openings between a detector 8 and a wafer 1, is arranged and the image of the wafer 1 is formed on the mask by a lens 9. On one hand, an SN ratio as a whole is improved extraordinarily because infrared rays radiated from a high temperature part are made incident to the detector 8 without damping almost only at the timing when the high temperature part is reached to the position of the minute opening. On this occasion, because the position of the minute opening corresponds to the position of each part on the wafer, the minute opening of the mask from which each date stored in a storage device 12 is obtained and the position of the wafer to which each data corresponds, are known by investigating preliminarily the position of the minute openings where the high temperature parts pass successively, accompanied by electron beam scanning.