Most of the existing copper indium gallium diselenide (CIGS) thin film solar cells are based on a cadmium sulfide (CdS) buffer layer fabricated using a chemical bath deposition (CBD) process. However, due to environmental pollution caused by material toxicity and the unique wet process's incompatibility with the vacuum process, many studies are now being actively carried out on nontoxic buffer layers. In this study, to replace CdS buffer layers, zinc sulfide (ZnS) buffer layers with a big band gap and a low optical loss at a short wavelength were fabricated using a magnetron sputtering system. For comparative analysis, this study also fabricated CdS buffer layers using the CBD process. Then, the conversion efficiency of CIGS thin film solar cells deposited with ZnS and CdS thin film as buffer layers was measured. The light conversion efficiency of ZnS buffer layer-based CIGS was measured at 14.44%, while that of the CdS buffer layer-based CIGS was measured at 15.71%. Given that both are higher than the minimum conversion efficiency required for commercialization (10%), ZnS buffer layer-based solar cells could have a competitive edge over the existing CdS buffer layer-based solar cells.