A porous element containing a fluoropolymer comprising vinylidene fluoride as a main unit and having a density of 0.55 1.30 g/cm.sup.3 and a Gurley value of not more than 150 sec/100 cc is used as a polymer substrate of a solid electrolyte to be placed between a positive electrode and a negative electrode. As a result, the solid electrolyte layer shows fine ion conductivity and an ion polymer secondary battery having strikingly improved low temperature characteristics, cycle characteristics and high-rate discharge characteristics as compared to conventional batteries can be obtained.