If interlayer cations of micas can move in the interlayer, the micas show ionic conductivity.However, K-type fluorophlogopite (KMg 3 AlSi 3 O 10 F 2 ) which is the representative synthetic mica and the main crystal of mica ceramics is known as an electrical insulator. If interlayer cations of micas are smaller than K + ion which is the interlayer cation of K-type fluorophlogopite and a larger amount of cation is stuffed in the interlayer, the micas have a higher possibility as ionic conductors. As one of such micas, highly charged sodium fluorophlogopite mica (Na 4 Mg 6 Al 4 Si 4 O 20 F 4 ·xH 2 O) which is called "Na-4-mica" is nominated. But there are no studies on its ionic conductivity. In this study, Na-4-mica could be synthesized by new and simple method which consisted of mixing of MgO, Al 2 O 3 , SiO 2 and NaF, calcination, sieving, compaction and firing in a sealed platinum container. The conductivity of the obtained Na-4-mica increased with an increase in temperature and reached 4.3×10 -4 S/cm at 650ºC.