PURPOSE: To prevent the destruction of an ion selective sensitive film by escaping the expanded air in a hollow tube to prevent the increase of the pressure therein when the temp. of a liquid to be tested is high.

CONSTITUTION: The inside liquid 12 and air 15 are heated to expand and the pressure in the hollow tube 6 increases when the ion selective sensitive film 13 attached to the hollow tube in a manner as to seal one end thereof is immersed into the liquid 14 to be tested. A stem 11 integral with an electrode 17 rises and the air in a cavity 18 is discharged through a hole 8 to the outside when the internal pressure rises higher than the frictional force between a packing 16 and the stem 11. The stem 11 is lowered by the pressure of the atm. flowing into the cavity 18 through the hole 8 when the temp. falls. Since the stem 11 moves upward or downward according to the ambient temp. in the above-mentioned manner, the difference between the internal pressure and external pressure of the tube 6 is decreased and the electrode is made usable over a wide temp. range.