A precursor composition capable to form a solid-state electrolyte having good ion conductivity and high heat-resistance, by simple curing procedure, comprising; (A) a polymerizable ion pair compound represented by formula I, (B) a cross-linker having two or more unsaturated groups co-polymerizable with the polymerizable ion pair compound (A), and (C) non-polymerizable ion pair compound. Each of R1, R2, R3, R4, R5 and R6 represents hydrogen atom, alkyl or aryl group having 1-12 carbon atoms; and each of R7 and R8 represents alkyl or aryl group having 1-12 carbon atoms. X designates either of halogen atom, AlCl.sub.4.sup.-, PF.sub.6.sup.-, BF.sub.4.sup.-, bis (oxalato) borate anion, R.sub.f1SO.sub.3.sup.-, (NC).sub.2N.sup.-, (R.sub.f2Y).sub.2N.sup.-, (R.sub.f2Y).sub.3C.sup.-, and (R.sub.f2Y) .sub.2C.sup.-Q, where R.sub.f1 designates perfluoro alkyl or aryl group having 1-8 carbon atoms; each R.sub.f2 designates fluorine atom, perfluoro alkyl or aryl group having 1-8 carbon atoms; Y designates --SO.sub.2--or --PO--R.sub.f2; and Q designates --H or --CO--R.sub.f2 ##STR00001##