The invention is directed at an ion-conductive solid polymer-forming composition, a binder resin and an ion-conductive solid polymer electrolyte comprising (A) a polymeric compound containing polyvinyl alcohol units and having an average degree of polymerization of at least 20, in which compound some or all of the hydroxyl groups on the polyvinyl alcohol units are substituted with oxyalkylene-containing groups to an average molar substitution of at least 0.3, (B) an ion-conductive salt, and (C) a compound having crosslinkable functional groups. The composition and the polymer electrolyte obtained therefrom have a high ionic conductivity and a high tackiness. Moreover, the polymer electrolyte has a semi-interpenetrating polymer network structure, giving it excellent shape retention.