By mating a main part with a first prism part or second prism part, a terahertz-wave spectrometer can easily switch between optical paths of a terahertz wave T propagating within a spectroscopic prism. When the main part mates with the first prism part, the terahertz wave T incident on an entrance surface passes through a depression, so as to be reflected by an arrangement part, whereby reflection spectrometry can be performed. When the main part mates with the second prism part, the terahertz wave T incident on the entrance surface is refracted by the depression, so as to pass through an object to be measured within a groove, whereby transmission spectrometry can be preformed.