A terahertz-wave spectrometer comprising: a light source for emitting laser light; a branching unit for splitting the laser light emitted from the light source into pump light and probe light; a terahertz-wave generator for generating a terahertz wave in response to the pump light incident thereon after branching off at the branching unit; a spectroscopic prism, having entrance and exit surfaces for the terahertz wave and an arrangement surface for an object to be measured, for propagating therewithin the terahertz wave incident on the entrance surface, causing the arrangement surface to reflect therethrough the terahertz wave, and then emitting the terahertz wave from the exit surface; and a terahertz-wave detector for receiving the terahertz wave emitted from the exit surface of the spectroscopic prism and the probe light branching off at the branching unit and detecting a correlation between the terahertz wave and the probe light; wherein, in the spectroscopic prism, a prism part (52, 62, 72, 82) including the arrangement surface is slidable with respect to a main part (51, 61, 71) in a direction intersecting the optical path of the terahertz wave transmitted through the spectroscopic prism, while a plurality of arrangement regions to be arranged with the object are provided in the arrangement surface in the prism part (52, 62, 72, 82) along a sliding direction, wherein the main part (51, 61, 71) comprises the entrance and the exit surfaces for the terahertz wave, wherein the main part (51, 61, 71) is provided with a groove (51a, 61a, 71a) into which the prism part (52, 62, 72, 82) including the arrangement surface is slidably fitted, characterized in that the arrangement surface is an upper face of the prism part (52, 62, 72, 82). a light source for emitting laser light; a branching unit for splitting the laser light emitted from the light source into pump light and probe light; a terahertz-wave generator for generating a terahertz wave in response to the pump light incident thereon after branching off at the branching unit; a spectroscopic prism, having entrance and exit surfaces for the terahertz wave and an arrangement surface for an object to be measured, for propagating therewithin the terahertz wave incident on the entrance surface, causing the arrangement surface to reflect therethrough the terahertz wave, and then emitting the terahertz wave from the exit surface; and a terahertz-wave detector for receiving the terahertz wave emitted from the exit surface of the spectroscopic prism and the probe light branching off at the branching unit and detecting a correlation between the terahertz wave and the probe light; wherein, in the spectroscopic prism, a prism part (52, 62, 72, 82) including the arrangement surface is slidable with respect to a main part (51, 61, 71) in a direction intersecting the optical path of the terahertz wave transmitted through the spectroscopic prism, while a plurality of arrangement regions to be arranged with the object are provided in the arrangement surface in the prism part (52, 62, 72, 82) along a sliding direction, wherein the main part (51, 61, 71) comprises the entrance and the exit surfaces for the terahertz wave, wherein the main part (51, 61, 71) is provided with a groove (51a, 61a, 71a) into which the prism part (52, 62, 72, 82) including the arrangement surface is slidably fitted, characterized in that the arrangement surface is an upper face of the prism part (52, 62, 72, 82).