Polyethylene and polypropylene, which have high transmittance in the terahertz band, are commonly used for terahertz lens systems. However, it is rather difficult to provide small lens systems, because the refractive indexes of polyethylene and polypropylene are small. In this research, we designed a small terahertz lens system by using a thin lens with a high-refractive index material, in addition to polyethylene or polypropylene.