TECHNOLOGY FOCUS - POLYMERS - Preferred Oxetane Compound: The oxetane compound is at least one type selected from formula (2) and formula (3).m3 and m4=1-6;n3 and n4=1-20; andR5-R8=1-12C alkyl which may be independent and branched.The oxetane compound of formula (2), is diethylene glycol bis oxetane (m3= 2, n3= 2, R5= R6= ethyl group), triethylene glycol bis oxetane (m3= 2, n3= 3, R5= R6= ethyl group), tetraethylene glycol bis oxetane (m3= 2, n3= 4, R5= R6= ethyl group). The oxetane compound of formula (3), is polyethylene glycol monomethyl ether oxetane (m4= 2, n4= 3, R7= ethyl group, R8= methyl group), polyethylene glycol mono isopropyl ether oxetane (m4= 2, n4= 3, R7= ethyl group, R8= isopropyl group), polyethylene glycol mono n-butyl ether oxetane (m4= 2, n4= 3, R7= ethyl group, R8= n-butyl group). The molar ratio of electrolyte salt to an ether chain of the oxetane compound having acrylate structure, is 0.01-0.9. Preferred Copolymer: The high molecular compound of formula (1) is 3-ethyl-3-hydroxy methyl oxetane (EHO)-methylmethacrylate (MMA)/methoxy polyethylene glycol acrylate (AM-90G) copolymer (R1= methyl group, R2= hydrogen atom, R3= ethyl group, R4= methyl group, m1, m2= 2, n1=1, n2= 9, a, b= 10-400). Preferred Method: The ring opening polymerization is performed by heating catalyst at 30-150 degrees C in the presence of the nonaqueous solvent. The catalyst is an electrolyte salt and/or a cation initiator. EXAMPLE - Trimethylol propane and diethyl carbonate, were reacted and 3-ethyl-3-hydroxymethyl oxetane (EHO) was obtained. Subsequently, transesterification of EHO was performed with methylmethacrylate (MMA). After reacting for 2 hours, EHO (in g) (232), MMA (200), oxidation dibutyltin (10) and 2,6-tert-butyl-4-methyl phenol (6), were added to a four neck flask and distillation was performed with methanol under reduced pressure. n-hexane was added to residue and distilled by water wash. The resulting product was dried at 100 degrees C and 2 mmHg, and 3-methacryloxy methyl-3-ethyl oxetane (a= 10-400) (EHO-MMA) with yield of 60% was obtained. EHO-MMA (2) and methoxy polyethylene glycol acrylate AM-90G (5), were added to diethylene glycol bis oxetane (DDOE) (25). 2,2'-azobis(isobutyronitrile) (0.5 mol%) was added as polymerization initiator to the reaction mixture and polymerization was performed at 80 degrees C under nitrogen atmosphere and EHO-MMA/AM-90G copolymer solution having number average molecular weight of 18000, and acrylate structure (which was a oxetane compound) was obtained. Lithium tetrafluoro boron (LiBF4) was added to nonaqueous solvent of polypropylene carbonate and dimethyl carbonate, and the amount was adjusted so that the weight ratio was set to 3 weight times of the copolymer solution. The resulting solution was added to the copolymer solution (3) consisting of oxetane compound and acrylate structure, with addition of polyethyleneglycol monomethyl ether oxetane (of formula (3), m4= 2, n4= 3, R7= ethyl group, R8= methyl group). The electrolyte salt amount was adjusted, so that the molar ratio of electrolyte salt to the ether chain of the oxetane compound was 0.4. The resulting solution was put into a Petri dish made of Teflon R and ring opening polymerization was performed at 70 degrees C under nitrogen atmosphere for 2 hours. Nonaqueous solvent was removed, then the residual amount was set to a predetermined amount at 70 degrees C and film of open ring polymer (cured substance) of oxetane compound was obtained. The obtained polyelectrolyte film had high flexibility and ion conductivity of 1 x 10-3 S/cm. The amount of nonaqueous solvent was 43 weight%.