The rate-determining step in electrochemical oxidation reaction of Li metal at the interface between an Li negative electrode and xLi2S–(100−x)P2S5 (x = 75, 70 or 67) solid electrolyte was determined on the basis of the study on Ag+ ion conductive solid electrolytes. For this purpose, potential step technique using a microelectrode was applied. Two possible rate-determining steps, the diffusion of Li+ ion vacancies and the reaction between interstitial Li+ ions and Li+ ion vacancies, were assumed, and we found the diffusion of Li+ ion vacancies is the rate-determining step in the present study. We also evaluated the exchange current density (i 0) of the Li/Li+ couple reaction and ionic conductivity (κ) of each solid electrolyte. Both i 0 and κ were increased as the x value was increased, suggesting that the conductivity of Li+ ions or the rate of the diffusion of Li+ ion vacancies was closely related to the rate of the Li/Li+ couple reaction.