PURPOSE: To simply, rapidly and accurately measure the concn. of a chemical solution during treatment, by a method wherein an auxiliary tank is provided in continuous relation to the last tank of a plurality of continuously provided treating tanks for continuously treating a steel strip while recirculation tanks are provided so as to be brought into communication with the treating tanks and a receiving tank is provided so as to be communicated with the auxiliary tank to detect the change in the liquid level of each chemical solution.

CONSTITUTION: Chemical solutions containing the same components are introduced into continuously provided treating tanks 11W13 and a single treating tank 14 and auxiliary tanks 21, 22 provided to the final stages are kept empty. A steel strip to be treated is continuously passed through the treating tanks 11W 13, the auxiliary tank 21, the treating tank 14 and the auxiliary tank 22 after washed with water prior to being introduced into the treating tanks 11, 14 and treated with the chemical solutions. Water and the chemical solutions are carried out and carried in as shown by arrows 60W68 by the steel strip and the changes in the amounts of the chemical solutions in the treating tanks 11W13 and the auxiliary tanks 21, 22 are detected by level meters 51W56 in recirculation tanks 31W34 and receiving tanks 41, 42 respectively communicated with the treating tanks and the auxiliary tanks as level change and the concn. of the chemical solution during treatment is rapidly and accurately calculated from the detected values by an operation apparatus.