Introduction of an interlayer between a cathode and a sulfide solid electrolyte is a well-known method ft reducing the interfacial resistance and improving the performance of all-solid-state batteries. However, tF mechanism responsible for the interlayer remains unclear because it is difficult to observe the reactions at tf nanometer-scale range. In this study, thin-film model interface of LiCoO2/80Li(2)S.20P(2)S(5) and LiCoO2/Li3PO4, 8OLi(2)S.20P(2)S(5) are fabricated by pulsed-laser deposition. The model interfaces are investigated by performin electrochemical measurements and depth-resolved X-ray absorption spectroscopy to clarify the effect of tt Li3PO4 interlayer. The results indicate that a reaction product layer forms between the LiCoO2 cathode and 80Li(2)S-20P(2)S(5) electrolyte during charge/discharge processes, resulting in high interfacial resistance. Meanwhile the formation of the reaction product layer can be suppressed by the introduction of a Li3PO4 interlayer.