We developed a new structural ion sensor for measuring sodium ions without using a reference electrode. The ion sensor, which is all solid state, has a structure of double electrodes, which are integrated on the base plate, inside the sodium ion sensitive membrane. The ion sensitivity of the sensor was 41.4 mV/decade in the range of 10-1 to 10-4M. To investigate the response mechanism of the potential between two electrodes, the impedance characteristic was measured using the electrochemical impedance method. The impedance of the ion sensitive membrane changed by changing the sodium ion concentration in solution. These results suggest a correlation between the membrane potential and the membrane impedance. In conclusion, the potential difference between the electrodes is caused by the difference in ion concentration of the electrodes. © 2013 The Institute of Electrical Engineers of Japan.