PURPOSE: To measure accurately the surface temperature of an object without being troubled by error factors neither lowering the temperature of the object, by receiving microwaves having a specific wavelength, which are radiated from the surface of the object, through an atmosphere where a flame of a heating burner or the like and floating solid materials exist and detecting the surface temperature of the object with a reference noise source.

CONSTITUTION: This device consists of a parabolic antenna 21 which is provided near a furnace wall 13 and is directed toward the surface of an object 10 whose temperature should be measured, an amplifier/square-law detector 16 for amplification and square- law detection of microwaves having a specific wavelength which are received by this antenna, a lock-in amplifier 17 which compares detected microwaves with microwaves from the reference noise source repeatedly and outputs the difference, and an operation recorder 18B which converts a relative intensity of obtained microwaves to a temperature and displays it. The parabolic antenna 21 is used for the purpose of improving the directivity of incident microwaves and making it possible to measure the temperature in a specific part of the surface of the object in the furnace. For the purpose of shutting out completely the influence of floating solid materials to perform processings following amplification advantageously, it is better that the wavelength of microwaves to be used is long.