Abstract: A crosslinked material of a polyether copolymer comprising: (A) 1 to 98% by mol of a repeating unit derived from a monomer represented by the formula (I): (B) 95 to 1% by mol of a repeating unit derived from a monomer represented by the formula (II): (C) 0.005 to 15% by mol of a repeating unit derived from a monomer having one epoxy group and at least one reactive functional group; provides a polymer solid electrolyte which is superior in ionic conductivity and also superior in processability, moldability, mechanical strength and flexibility, furthermore thermal resistance.