A measuring apparatus measures a time-domain waveform of THz wave pulse by time-domain spectroscopy. The apparatus includes a detector for detecting THz wave pulse containing a signal component at frequency fs, a small signal detector for detecting the signal component at frequency fs of a signal supplied from the detecting portion, with a time constant τ, and an integrator. The integrator is connected to an output stage of the small signal detector to integrate signals supplied from the small signal detector, and has a time constant below the time constant τ and over 1/(2fs).