PURPOSE: To stabilize the potential between liquids and to make measurement with high accuracy by dipping one of ion electrodes into the internal liquid sealed hermetically by a solid salt bridge consisting of a molten mixture composed of KCl and AgCl.

CONSTITUTION: The solid salt bridge 8 is constituted by molding the melt of KCl and AgCl (the ratio of the KCl and AgCl is about 8:10) then coating the surface with a hydrophobic resin and applying thermal impact to the molding thereby forming microcracks thereto. The ion electrodes 4, 5 are constituted by providing the same ion selective field effect transistors on the same substrate 6. The gate G of the electrode 4 contacts directly with the liquid A to be inspected and the gate G' of the electrode 5 contacts with the internal liquid B sealed hermetically by the bridge 8 and an epoxy resin 7. The two electrodes 4, 5 are connected to reverse polarities and the difference between the outputs from both electrodes is a measuring output. The contamination of the internal liquid by the liquid to be inspected is thus eliminated and since a small amt. of the internal liquid is required, the reduction in the size of the instrument is possible and since the solid salt bridge acts as a kind of battery, the potential between the liquids is stabilized.