Specifications of two measurement systems, free space measurement using a vector network analyzer and low-cost-type terahertz time-domain spectroscopy using a multimode laser diode, have been compared in the frequency region of millimeter/sub-THz waves. In the comparison, accuracy, cost, measurement time, calculation time, etc. were considered. Four samples (Rexolite, RO3003, Ultralam 3850HT-design, and L1000HF) were selected for the comparison of the specifications of the two methods. The acquired data was used to compute the complex permittivity of measured materials. The extracted results by free space measurement agreed well to the ones obtained by low-cost-type terahertz time-domain spectroscopy. This result proves free space measurement that can be assessed as a new method of material characterization in the sub-THz region successfully worked. Furthermore, free space measurement was proved to be suitable for a measurement in a narrow frequency range. On the other hand, low-cost-type terahertz time-domain spectroscopy has features not only low cost but also measurement capability in wide frequency range.