Abstract: An apparatus for measuring a concentration of a specific ion contained in a test liquid held in a container with the aid of a reference electrode and an ion sensitive field effect transistor having a gate portion selectively sensitive to the specific ion, including a series circuit of a reference resistor and a constant voltage supply source connected across drain and source of the ion sensitive field effect transistor, a potential control circuit having inputs connected across the reference resistor to detect a potential difference across the reference resistor for controlling a source or drain potential of the ion sensitive field effect transistor in such a manner that the potential difference remains at a predetermined value and a voltmeter for measuring the source or drain potential as a measure of the ion concentration.