For the development of novel non-aqueous sodium ion conductor with safety of sodium secondary cell, non-flammable ionic liquid is attractive as electrolyte component. A preliminary study has been carried out for the purpose of constructing sodium ion conducting electrolyte based on ionic liquid. The solubility of sodium salt such as NaBF4 in ionic liquid is poor, thus the ternary electrolyte has been prepared where NaBF4 with poly(ethylene glycol) dimethyl ether (PEGDME) as coordination former is dissolved with ionic liquid diethyl methoxyethyl ammonium tetrafluoroborate (DEMEBF4). The maximum conductivity among the prepared solutions, ca. 1.2mScm−1 at 25°C, was obtained when the molar ratio (ethylene oxide unit in PEGDME):NaBF4:DEMEBF4 was 8:1:2. The relationship between the conductivity of the ternary electrolyte and its solution structure has been discussed.