A composite Na+ conductor of polycrystalline NASICON mixed with glassy Na2O-Al2O3-4SiO(2) (40 wt%) was investigated and used for a Cl-2 gas sensor. The densification of the solid electrolyte was improved by the addition of the glassy material. The sensor probe using the composite electrolyte and RuO2 + NaCl (1:1) measuring electrode produced a rapid electromotive force (EMF) change for the Cl-2 gas concentration below 10 ppm at above 450 degreesC. For the low temperature of 400 degreesC, a slow EMF response was obtained due to the side reaction between the Cl-2, gas and Na2O on the solid electrolyte and/or slow Cl-2, reduction to Cl- ion on the electrode. This sensor EMF value in air was only slightly influenced by the changing of the O-2 or CO2 gas concentration.