PROBLEM TO BE SOLVED: To provide a prism member capable of improving measurement accuracy of spectroscope regardless of a shape of an object to be measured, a terahertz wave spectroscopic measurement device using the same, and a terahertz wave spectroscopic measurement method.SOLUTION: In a terahertz wave spectroscopic measurement apparatus 1, a recessed part 51a comprising a placement part 31c of a spectral prism 31 is filled up with an object 34 to be measured, and a recessed part 51a is filled up with liquid 50 showing insolubility. Therefore, in a flow path of a terahertz wave T passing the placement part 31c, it can be avoided that the terahertz wave T passes in the air. Consequently, influences of absorption by moisture in the air and influences of reflection loss on a first refraction surface 52 and a second refraction surface 53 can be eliminated, and measurement accuracy of spectroscopy can be improved. A shape of the recessed part 51a is changeable in many ways in accordance with a shape or the like of the object 34 to be measured, and since the recessed part 51a can be easily filled up with the liquid 50 even when the shape differs, convenience in measurement is maintained.
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