Molecular design of elastomeric polymer solid electrolytes was presented, whose concept was based on the rubbery state of polymer. For the ionic conduction, elastomeric amorphous poly(oxyethylene) (POE) matrixes were produced by the copolymerization technique. At first, high molecular mass copolymers were synthesized by copolymerization of ethylene oxide (EO) with epichlorohydrin (EH), and the ionic conductivity was evaluated for the samples doped with lithium perchlorate. The EH units, however, did not contribute to the ion conduction, although the EH units were effective for preventing the crystallization of POE segments. Next, high molecular mass comb-shaped POEs with oxyethylene segments as side chains were synthesized. Both main chain and side chains were oxyethylene units, and the molecular mass was on the order of 10(6). The oxyethylene side chains were found to be useful for solvating the salt and the preventing the crystallization of POE segments. Consequently, the high molecular mass comb-shaped POEs with oxyethylene side chains showed the higher ionic conductivity when doped with lithium perchlorate or lithium bis(trifluoromethanesulfonyl)imide. The structure and mechanical properties of comb-shaped POEs were also reported, and the usefulness of the concept for the molecular design of elastomeric polymer solid electrolytes was displayed.