PURPOSE: To enable rapid and exact measurement by subjecting standard pH liquids to a heat exchange with a liquid to be measured and calibrating a pH meter with the standard pH liquids which are kept at the same liquid temp. as the temp. of the liquid to be measured, then measuring the pH value of the liquid to be measured at the present liquid temp.

CONSTITUTION: The plating liquid in a plating cell 2 for continuous plating of a steel sheet 1 is supplied from a plating liquid tank 3. The standard liquids X1WXn such as phthalate soln. (4pH), neutral phosphate soln. (7pH) and borate soln. (9pH) are housed in respective standard pH liquid tanks liquids X1WXn. The calibration of the pH meter is executed at every prescribed time. The liquid temps. of the respective liquids X1WXn at the time of the measurement are measured by a thermometer contg. a pH meter, etc., and the pH of the standard liquids is exactly determined. The pH of the liquid to be measured and the potential difference E of the pH meter 6 are calibrated by using the pH values pH1WpHn of the respective liquids X1WXn determined in such a manner and the measured inter-electrode potential differences E1WEn, by which zero adjustment, etc., are completed. All valves V0, V1WVn are closed and valves Va, Va are controlled, upon completion of the calibration, by which a part of the flow rate of the plating liquid flowing in a pipe 3a is injected into a measuring tank 5 and the temp., etc., of the plating liquid are measured.