PROBLEM TO BE SOLVED: To provide a dielectric refraction index detection method capable of highly accurately detecting a refraction index of a dielectric or the like in a case of detecting the refraction index of the dielectric or the like by reflection of an electromagnetic wave.SOLUTION: A terahertz wave is incident perpendicularly on a rough surface 13A of a dielectric 13 via a lens 8, and an intensity signal I representing intensity of the terahertz wave reflected by the rough surface 13A is detected. On the other hand, the terahertz wave is incident perpendicularly on a mirror surface 12A of a metallic plate 12 via the lens 8, and an intensity signal Irepresenting intensity of the terahertz wave reflected by the mirror surface 12A is detected. Furthermore, the intensity signals I and Iare each detected at a plurality of viewing angles &phgr; of the lens 8, respectively, a ratio (I/I) of the intensity signal I to the intensity signal Iat each viewing angle &phgr; is obtained and a characteristic curve rep
