Provided is a material which exhibits excellent ion conductivity and excellent processability and which can provide an electrolyte that exhibits excellent water-resistant shape retention properties after processing. A polyether copolymer having polyether segment blocks having cationic groups and hydrophobic polyether segment blocks. The polyether segments having cationic groups preferably have oxirane monomer units represented by general formula (1). The polyether copolymer may have oxirane monomer units having crosslinkable groups. An electrolyte is obtained by crosslinking a composition containing the polyether copolymer and a crosslinking agent. (In general formula (1), A+ denotes a group having an onium cation structure having a cationic nitrogen atom, and X− denotes an anion).