A novel processing technique for the preparation of Na+ super ionic conductor (NASICON: Na3Zr2Si2PO12) thin films have been developed using an aqueous complex precursor derived from hydroxyacid-added aqueous solutions of all-inorganic materials. Spinnable viscous gel was obtained from a tartaric acid (TA)-added aqueous solution after holding the temperature at 2 degrees C for 7 days. NASICON thin films could be fabricated onto alumina substrates by spin-coating the TA-based viscous gel, drying at 150 degrees C, and sintering at 1000 degrees C for 3 h.