The influence of moisture in a test gas containing Cl-2 was investigated for a Cl-2 gas sensor using a NaCl-RuO2 mixed measuring electrode and a composite Na+ conductor of sodium superionic conductor, 40 wt % (Na2O-Al2O3-4SiO(2)). For dried Cl-2 gas, the electron number (n) for the reaction on the electrodes was 1.86 which was very close to the theoretical value n=2.0. When water vapor was added to the Cl-2 measuring gas, the electromotive force (emf) significantly decreased, since the Cl-2 gas concentration was reduced by the reaction with the H2O gas. Based on the calibration curve observed from the relationship between emf and Cl-2 concentration at 450degreesC under dry conditions, it is suggested that the sensor can determine parts per billion levels of Cl-2 gas under ambient conditions. Experimentally, the observed response time became faster with an increase in the flow rate and mainly depended on the exchange time of the test gas.