We have developed a wide frequency tunable Continuous Wave (CW) Terahertz (THz) Signal Generator (SG) on the principle of Difference Frequency Generation (DFG) in a Gallium Phosphide (GaP) crystal. The CW THz SG has the merits of high resolution, high accuracy, wide dynamic range, high stability, high durability, easy operation/maintenance, and low cost. Combining the CW THz SG as a light source with a superconducting Transition Edge Sensor (TES) bolometer cooled by a pulse tube refrigerator, the spectrometer working as a non-stop system could be realized. As we could have increased the THz output power up to 0.1 μW at the sacrifice of lack of frequency resolution, it is now possible to apply a pyroelectric detector or a bolometer camera at room temperature operation for spectrometer or monochromatic imaging. Accurate THz spectrometer could be used for evaluation of organic crystals like as pharmaceuticals. One of the reasons for underutilization of THz spectroscopy is that most of the absorptions observed in THz range are not assigned to vibrational modes. We have developed a technique to clarify them by comparing polarization dependent spectra with Density Function Theory (DFT) calculation results for pharmaceutical single crystals.