In a terahertz-wave spectrometer, a spectroscopic prism is provided with a prism part slidable with respect to a main part thereof. Along the sliding direction, an arrangement surface in an upper face of the prism part is provided with a plurality of arrangement regions K to be arranged with objects to be measured. Therefore, after completing the measurement of optical constants for one object, the prism part is slid, so as to shift the next object onto an optical path of a terahertz wave T, whereby a plurality of objects can be measured smoothly without cleaning the arrangement surface.