Ceramic sheet of Na+ ion-conducting solid electrolyte, Na3Zr2Si2PO12 (NASICON), has been prepared by tape-casting method. Addition of 60Na2O-10Nb2O5-30P2O5 glass as a sintering aid is crucial for fabrication of fully-densified impermeable sheet. Otherwise the gas tightness is not achieved even sintered at 1200 °C. The best preparation condition is 5 or 10 wt% grass-addition with respect to NASICON and sintering at 1000–1100 °C, which is lower by 100–200 °C than the typical densification temperature of NASICON without the additive. A sample with a thickness of 35 μm and total conductivity of 4.4 × 10−4 S cm−1 is obtained by 5 wt% glass addition and sintering at 1100 °C for 4 h.