A prism member having an entrance surface for arranging a terahertz-wave generator for generating a terahertz wave in response to pump light incident thereon, an arrangement part for arranging an object to be measured, an exit surface for arranging a terahertz-wave detector for detecting a correlation between the terahertz wave transmitted through the object in the arrangement part and probe light, a first optical surface for collimating or condensing the terahertz wave incident thereon from the entrance surface toward the arrangement part, and a second optical surface for condensing the terahertz wave transmitted through the arrangement part toward the exit surface, the arrangement part forms a depression adapted to be filled with a liquid incapable of dissolving the object therein.