Ionic conductivity and mechanical properties of a mixed polymer matrix consisting of poly(ethylene glycol) (PEG) and cyanoresin type M (CRM) with various lithium salts and plasticizer were examined. The CRM used was a copolymer of cyanoethyl pullulan and cyanoethyl poly(vinyl alcohol) with a molar ratio of 1:1, mixed plasticizer was ethylene carbonate (EC) and propylene carbonate (PC) at a volume ratio of 1:1. The conductive behavior of polymer electrolytes in the temperature range of $298{sim}338;K$ was investigated. The $PEG/LiClO_4$ complexes exhibited the highest ionic conductivity of ${sim}10^{-5}S/cm$ at $25^{circ}C$ with the salt concentration of 1.5 M. In addition, the plasticized $PEG/LiClO_4$ complexes exhibited improvement of ionic conductivity. However, their complexes showed decreased mechanical properties. The improvement of ionic conductivity and mechanical properties could be obtained from the polymer electrolytes by using CRM. The highest ionic conductivity of PEG/CRM/$LiClO_4$/(EC-PC) was $5.33{ ime}10^{-4}S/cm$ at $25^{circ}C$.