A reflection-type terahertz time domain spectrometer, comprising: an ultrashort light pulse source which generates an ultrashort laser light pulse a light split means which splits said ultrashort laser light pulse which is generated from said ultrashort light pulse source, a terahertz waves generating means which is excited by one ultrashort laser light pulse that is split by said light split means to generate terahertz waves, an input optical path through which said terahertz waves that are generated by said terahertz waves generating means are propagated, an irradiating means which irradiates a sample with said terahertz waves propagated through said input optical path, an output optical path through which terahertz waves having exited from said irradiating means are propagated, an optical delay means (18,19) which controls time delay of the other ultrashort laser light pulse which is split by said light split means, and a detecting means which receives and detects said terahertz waves propagated through said output optical path in the condition that the other ultrashort laser light pulse whose time delay is controlled by said optical delay means is considered as a probe light, characterized in that said irradiating means has at least one planar interface near which said sample is arranged and a refractive index greater than that of a peripheral region contacting said planar interface and is disposed between said input optical path and said output optical path such that said incident terahertz waves which are incident in parallel with said planar interface undergo total internal reflection at said planar interface and terahertz waves which are parallel with said planar interface exit in symmetry with said incident terahertz waves at the total reflection point, wherein the reflection-type terahertz time domain spectrometer further comprises a polarization control means which controls polarization of said terahertz waves in the midst of said input optical path. an ultrashort light pulse source which generates an ultrashort laser light pulse a light split means which splits said ultrashort laser light pulse which is generated from said ultrashort light pulse source, a terahertz waves generating means which is excited by one ultrashort laser light pulse that is split by said light split means to generate terahertz waves, an input optical path through which said terahertz waves that are generated by said terahertz waves generating means are propagated, an irradiating means which irradiates a sample with said terahertz waves propagated through said input optical path, an output optical path through which terahertz waves having exited from said irradiating means are propagated, an optical delay means (18,19) which controls time delay of the other ultrashort laser light pulse which is split by said light split means, and a detecting means which receives and detects said terahertz waves propagated through said output optical path in the condition that the other ultrashort laser light pulse whose time delay is controlled by said optical delay means is considered as a probe light, characterized in that said irradiating means has at least one planar interface near which said sample is arranged and a refractive index greater than that of a peripheral region contacting said planar interface and is disposed between said input optical path and said output optical path such that said incident terahertz waves which are incident in parallel with said planar interface undergo total internal reflection at said planar interface and terahertz waves which are parallel with said planar interface exit in symmetry with said incident terahertz waves at the total reflection point, wherein the reflection-type terahertz time domain spectrometer further comprises a polarization control means which controls polarization of said terahertz waves in the midst of said input optical path.