The high-power terahertz time-domain spectroscopy (THz-TDS) has been developed on the basis of a compact S-band electron linac at AIST, Japan. It is strongly expected for inspection of dangerous materials in the homeland security field. THz radiations are generated in two methods with the ultra-short bunch. One is THz coherent synchrotron radiation (THz-CSR). The other is THz coherent transition radiation (THz-CTR). In the preliminary experiment to obtain the characteristics of THz-CTR, it was observed that the focused THz-CTR had the donut profile in the transverse field due to its initial radial polarization. It was considered to be z-polarization at the focus point. In case of the THz-TDS experiment, THz-CTR was controlled to be linearly polarized with the polarizer and focused to an electro optical (EO) crystal to obtain a THz temporal waveform using EO sampling method. It leads to THz spectrum with Fourier transform. In this paper, we will describe details of our linac and results of the THz-CTR-TDS experiment.