Novel polymeric gel electrolytes consisting of poly(ethylene oxide)–modified poly(methacrylate) (PEO–PMA) with magnesium imide (Mg[(CF3SO2)2N]2) as the electrolytic salt and mixed alkyl carbonates as the plasticizer have been prepared by photo-induced radical polymerization. The polymeric gel film was flexible and self-standing with proper mechanical strength. The ionic conductivity of the polymeric gel film was about 10−3 S cm−1 at room temperature. The ionic conductivity increased with the content of the plasticizer, ethylene carbonate (EC) and dimethyl carbonate (DMC), in the complex, while the mechanical strength of the gel film decreased with the increase in the plasticizer. The highest conductivity was obtained for the composition of 75 wt.% of the plasticizing component, EC+DMC dissolving Mg[(CF3SO2)2N]2 in the gel. The applicability of the present gel film to a rechargeable battery system was examined by a prototype cell consisting of Mg-doped V2O5 and V2O5 (or MnO2) as the negative and positive electrodes, respectively.