A compact stabilized zirconia-based planar-type solid state electrochemical gas sensor using columbite type MnNb₂O₆sensing electrode (SE) was designed and developed, aiming at monitoring low concentration of SO₂at high temperature. Among the different sensing electrode materials calcinated at 800, 1000 and 1200℃, the device utilizing MnNb₂O₆-SE sintered at 1000℃ displayed the highest response of -27 mV to 5 ppm SO₂and the low detection limit of 50 ppb at 700℃. A linear dependence of AV for the sensor attached with MnNb₂O₆-SE sintered at 1000℃ on the logarithm of SO₂concentration in the range of 0.05-5 ppm at an operating temperature of 700℃ was achieved, which the sensitivity was -13 mV/decade. The present sensor showed fast response time of 37 s to 1 ppm SO2 at 700 ℃. Furthermore, the present device also displayed good continuous response and recovery, high temperature stability and selectivity toward various interfering gases at 700 ℃. Additionally, the sensing mechanism related to mixed potential was proposed and further demonstrated based on the measurement of polarization curve.