A cascade nitrile compound ([CH2N(CH2CH2CN)2]2, ED4CN) made by addition of acrylonitrile to alkyldiamine (1,2-diaminoethane), has been used as a plasticizer for solid polymer electrolytes. The ionic conductivity of a polymer electrolyte using this type of plasticizer in polyethylene oxide (PEO)– and polyacrylonitrile (PAN)–LiClO4 complex was measured. Addition of ED4CN to PEO-based electrolytes did not enhance the conductivity of them. However, interaction between ED4CN and lithium ions in the complex was confirmed by infrared spectroscopy. The peak assigned to the stretching vibration of nitrile group in ED4CN shifted to high-energy side. The shift indicated that the nitrile groups interacted with the lithium ions in the PEO-based electrolytes. Conductivity enhancement was observed in the PAN-based electrolytes containing ED4CN. Conductivity of the electrolyte containing ED4CN was about 10 or 23 times larger than that of the electrolyte without ED4CN. Addition of ED4CN to a PAN–LiClO4 electrolyte decreases the glass transition temperature of the complexes. Conductivity enhancement of the PAN-based electrolyte with ED4CN containing lithium salt in high concentration was also confirmed. Other low molecular weight additives, tetraethylsulfamide (TESA) and a cascade nitrile compound, ([CH2CH2N(CH2CH2CN)2]2, TE4CN) were also used and their possibility for a conducting enhancer of PAN-based electrolytes was tested. TESA was effective; however, TE4CN was inactive for a conductance enhancer of the PAN-based electrolytes.