Mini-EUSO ("UV atmosphere" in Russian Space Program) is a science mission to observe UV phenomena in the night Earth atmosphere from a UV transparent porthole of the International Space Station and will serve as a pathfinder for future ultra-high energy cosmic ray missions such as KLYPVE-EUSO (K-EUSO) and JEM-EUSO. The Mini-EUSO optical design has two 250 mm double-sided Fresnel lenses and a flat focal surface. The overall length of the optical system is 300 mm. Its field of view is about $\pm22^{\circ}$. The RMS spot sizes between $0^{\circ}$ and $22^{\circ}$ field angle are 2.0 mm $\sim$ 3.5 mm in diameter, and conform to the pixel size of the M64 multi-anode photomultiplier (HAMAMATSU Photonics). The material of lenses is UV transparent PMMA (Poly Methyl Methacrylate, PMMA-000, MITSUBISHI RAYON) and the lens thickness is 11 mm. The lenses were manufactured using the diamond turning machine by the Materials Fabrication Laboratory of RIKEN. We confirmed that the surface roughness of lenses is less than 20 nm (RMS). In this paper, we will describe the details of the optical system and test results.