PURPOSE: To provide an ion concentration sensor which is provided with the performance demanded as reference electrode and can reduce the dimension by using a solid reference electrode which can reduce dimension.

CONSTITUTION: An ion concentration sensor is formed by connecting an ion sensitive electric field effect type transistor(ISFET) 7 which is formed by forming an ion sensitive film 5 on an insulating gate film 2 and a solid reference electrode 6 consisting of palladium hydride through an electric power sorce circuit F. Since the solid reference electrode is used, the stability for a long period and reproducibility can be improved, and the reduction of dimension is enabled. Further, the dimension of the ion concentration sensor itself can be reduced, and a variety of ion ISFETs can be made complicated, and the sensor which can measure the ion cencentration in plural kinds can be obtained.