Sodium superionic conductors in the system Na2O-Y2O3-P2O5-SiO2 were prepared by sol-gel method : humidity-controlled air or N-2 was passed into the reaction mixture to avoid precipitates such as nitrate and carbonate. Highly homogeneous xerogel was obtained by supplying water at a rate of about 1.8 x 10(-4) mol min(-1). The sintered samples of the xerogels with the composition between x = 5.33 and 5.60 in NaxY0.8P0.4Si3.6O12.0 showed a single crystal phase with the superionic conduction. The sample with the composition of x = 5.33 showed highly ionic conductivity of rho(300) = 13.9 Omega cm at 300 degrees C.