A polymer electrolyte-forming composition containing (A) a quaternary ammonium salt of general formula (1) below and (B) an ionic liquid can be converted into a polymer without compromising the excellent properties of the ionic liquid, thus enabling an electrolyte having an excellent safety and electrical conductivity and also a broad potential window to be obtained. ##STR00001## In formula (1), R.sup.1 to R.sup.3 are each independently an alkyl group of 1 to 5 carbons or a substituent having a reactive unsaturated bond and any two from among R.sup.1 to R.sup.3 may together form a ring, and R.sup.4 is methyl, ethyl or a substituent having a reactive unsaturated bond, with the proviso that at least one of R.sup.1 to R.sup.4 is a substituent having a reactive unsaturated bond. X is a monovalent anion, the letter m is an integer from 1 to 8, and the letter n is an integer from 1 to 4.