A method for accurately performing concentration measurements while compensating the dilution ratio as needed during continuous measurement of specified ingredient in the liquid to be tested. The liquid to be tested is diluted by a diluent, wherein the liquid to be tested and the diluent or the mixture of the two and the diluent is transferred by means of the respective tube pumps to a measuring section. Either or both of the quantities of liquid-feeding of those two liquids are changed at the beginning of measuring or in an optional stage during the time of the measurement, diluted and mixed at two or more dilution ratios. Then the real dilution ratio at the time of changing the quantities of the said liquids is found on the basis of the measured value of the substance to be measured at the respective dilution ratios, and the concentration of the substance contained in the liquid to be tested is determined by using the real dilution ratio.