PURPOSE: To make it possible to measure a wide range of solution concentration by seeking relative difference of potentials of the solutions that constitute a pair of half batteries.

CONSTITUTION: A solution B to be inspected is poured into a measurement tank 10, and a measurement electrode A is immersed in this solution B to form a half battery on one side. Further, a reference solution B' is filled in a reference tank 20, and a reference electrode A' is immersed in the reference solution B' to form a half battery on the other side. This reference tank 20 is immersed in the solution B to be inspected, and the potential difference between the two half batteries is measured with a potential measurement device D. This potential difference is generated in accordance with the temperature difference between the reference solution B' and the solution to be inspected.