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 part for an object to be measured, for propagating therewithin the terahertz wave incident on the entrance surface and 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 the spectroscopic prism comprises: a main part (51, 81, 111) having the entrance and exit surfaces, a depression (51a, 81a, 111a) formed on an upper face side, a first optical surface for collimating or converging the terahertz wave toward the depression, and a second optical surface for converging the terahertz wave having passed through the depression (51a, 81a, 111a) toward the exit surface; a first prism part (61, 91, 121), formed by a member having a refractive index substantially equal to that of the main part (51, 81, 111), including a projection adapted to detachably mate with the depression (51a, 81a, 111a) and having an upper face provided with the arrangement part for the object; and a second prism part (71, 101, 131), formed by a member having a refractive index lower than that of the main part (51, 81, 111), including a projection adapted to detachably mate with the depression (51a, 81a, 111a) and having a groove to become the arrangement part for the object, the groove being formed from an upper face toward a leading end of the projection; wherein the terahertz wave incident on the entrance surface is transmitted through the depression (51a, 81a, 111a) and reflected by the arrangement part when the first prism part (61, 91, 121) mates with the main part (51, 81, 111), but is refracted by the depression (51a, 81a, 111a) and transmitted through the groove when the second prism part (71, 101, 131) mates with the main part (51, 81, 111). 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 part for an object to be measured, for propagating therewithin the terahertz wave incident on the entrance surface and 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 the spectroscopic prism comprises: a main part (51, 81, 111) having the entrance and exit surfaces, a depression (51a, 81a, 111a) formed on an upper face side, a first optical surface for collimating or converging the terahertz wave toward the depression, and a second optical surface for converging the terahertz wave having passed through the depression (51a, 81a, 111a) toward the exit surface; a first prism part (61, 91, 121), formed by a member having a refractive index substantially equal to that of the main part (51, 81, 111), including a projection adapted to detachably mate with the depression (51a, 81a, 111a) and having an upper face provided with the arrangement part for the object; and a second prism part (71, 101, 131), formed by a member having a refractive index lower than that of the main part (51, 81, 111), including a projection adapted to detachably mate with the depression (51a, 81a, 111a) and having a groove to become the arrangement part for the object, the groove being formed from an upper face toward a leading end of the projection; a main part (51, 81, 111) having the entrance and exit surfaces, a depression (51a, 81a, 111a) formed on an upper face side, a first optical surface for collimating or converging the terahertz wave toward the depression, and a second optical surface for converging the terahertz wave having passed through the depression (51a, 81a, 111a) toward the exit surface; a first prism part (61, 91, 121), formed by a member having a refractive index substantially equal to that of the main part (51, 81, 111), including a projection adapted to detachably mate with the depression (51a, 81a, 111a) and having an upper face provided with the arrangement part for the object; and a second prism part (71, 101, 131), formed by a member having a refractive index lower than that of the main part (51, 81, 111), including a projection adapted to detachably mate with the depression (51a, 81a, 111a) and having a groove to become the arrangement part for the object, the groove being formed from an upper face toward a leading end of the projection; wherein the terahertz wave incident on the entrance surface is transmitted through the depression (51a, 81a, 111a) and reflected by the arrangement part when the first prism part (61, 91, 121) mates with the main part (51, 81, 111), but is refracted by the depression (51a, 81a, 111a) and transmitted through the groove when the second prism part (71, 101, 131) mates with the main part (51, 81, 111).