A total reflection terahertz wave measuring apparatus 1 is configured to acquire information on a subject S by a total reflection measurement method by use of a terahertz wave, and includes a light source 11, a branching part 12, a chopper 13, an optical path length difference adjusting part 14, a polarizer 15, a separator 17, a terahertz wave generating element 20, an internal total reflection prism 31, a terahertz wave detecting element 40, a ¼ wavelength plate 51, a polarization split element 52, a photodetector 53A, a photodetector 53B, a differential amplifier 54, and a lock-in amplifier 55. The internal total reflection prism 31 is a so-called aplanatic prism, and has an entrance plane 31a, an exit plane 31b, and a reflection plane 31c. The terahertz wave generating element 20 is provided to be integrated with the entrance plane 31a of the internal total reflection prism 31, and the terahertz wave detecting element 40 is provided to be integrated with the exit plane 31b of the internal total reflection prism 31.