The dielectric properties in the THz band can be measured by THz time-domain spectroscopy (THz-TDS). This method measures the time response of transmissions or reflections after ultrashort pulse electromagnetic waves including THz components enter a sample. One of the features of this method is the application of Fourier transformto measured time responseto gain the amplitudes and phase spectrums of the electrical field at the same time, thereby easily enabling the gaining of complex permittivity spectrums. In the course of this research, our group has developed reflective THz time-domain spectroscopic ellipsometry (THz-TDSE) in order to measure highlydoped samples, thin film samples, etc. Figure 1 shows the outline of the system which was developed by our group and Fig. 2 shows a photo of the system's equipment. Terahertz light is generated by an emitter used as an excitation light source which is a femtosecond laser pulsedand the wire-grid polarizer picks up only 45° linear polarized components, which enter a sample at incident angles between 70 and 80°. The elliptically polarized light reflected from the sample enters the wire-grid placed in front of a detector, where the angle of rotation switches to 0° or 90° to gain time response of s- and p- polarization reflections individually. Afterwards, through Fourier transform, amplitude reflection coefficient ratios and phase differences are gained, leading to complex permittivity.