Supercritical carbon dioxide (scCO2) as a treatment medium has a possibility to realize excellent room temperature conductivity more than 10−4 S/cm for polymer electrolytes in the dry state. In this study, a typical high ion-conductive polyether-based electrolyte which consists of poly-[ethylene oxide-co-2-(2-methoxyethoxy)ethyl glycidyl ether] (P(EO/EM)) and lithium perchlorate (LiClO4) was used as a model sample for the scCO2 treatment. We found the suitable scCO2 treatment conditions (pressure, temperature and time) for high conductivity. The conductivity of sample treated at 7.5MPa and 40°C for 40min was more than 100-times higher than that of original without the treatment, and the value decreased only 20% after 30days. DSC measurement revealed that the decrease in glass transition temperature (T g) is caused by the scCO2-treatment. The change of ionic association in the scCO2-treated samples was confirmed using FT-IR measurement. The scCO2 treatment gave rise to increase in peak fraction of free ClO4 − anions (620–625cm−1) and peak shift of ν(C–O–C) mode to lower frequency region (1060–1070cm−1) depending on ether–Li+ interactions.