Abstract: Using a synthetic diamond as the material of a prism allows ATR spectral analysis to be conducted only by pushing it into contact with a measuring object for measurement at places with poor environment, measurement of structures themselves, or measurement of samples sticking to cloths or the like, an optical part such as an infrared microscope system oriented sample plate can withstand a large number of times of use. A measuring instrument employs an attenuated total reflection (ATR) prism 14 of the synthetic diamond, having a nitrogen content of not more than 3 ppm and a boron content of not more than 3 ppm as contained in crystal. In combination with this prism 14 are optical-use mirrors 12, 17, lenses 13, 16, or optical fiber, whereby measurement can be conducted by pushing the prism into direct contact with a sample 15 without using a holder or the like.