A method for estimating a physical property of an aqueous sample containing a predetermined concentration of surfactant and water, comprising: preparing a standard sample by adding an oil component and optionally a solvent to a precursor sample containing water and a surfactant with a known Griffin's HLB value, the concentration of the surfactant with a known Griffin's HLB value being the same as the concentration of the surfactant in the aqueous sample; adding to the standard sample a phase-changing probe consisting of a compound having a different structure than that of the surfactant with a known Griffin's HLB value to quantify an amount of the phase-changing probe required until a phase structure of the standard sample changes; performing this quantification for a plurality of surfactants with different Griffin's HLB values under the same conditions to obtain a plurality of quantitative values; and preparing a calibration curve from the quantitative values and the HLB values of the surfactants included in the standard samples that provide the quantitative values, preparing a measurement sample by adding to the aqueous sample the same oil component and optional solvent as those added to the precursor sample in the same respective amounts as those in the precursor sample; adding to the measurement sample the same phase-changing probe as the one used for the measurement of the quantitative values of the calibration curve; and quantifying an amount of the phase-changing probe required until a phase structure of the measurement sample changes, and with values of the HLB value in the calibration curve defined as liquid physical property indexes, estimating the physical property of the aqueous sample from a liquid physical property index corresponding to a quantitative value of the phase-changing probe for the measurement sample; wherein the phase-changing probe is a compound selected from polyoxyethylene alkyl ether, polyoxypropylene alkyl ether, monoalkyl monoglyceryl ether, alkyl polyglyceryl ether, polyalkyl glycoside, polyoxyalkylene alkylamine, polyoxyalkylene fatty acid ethanol amide and polyoxyethylene sorbitan alkyl ester. preparing a standard sample by adding an oil component and optionally a solvent to a precursor sample containing water and a surfactant with a known Griffin's HLB value, the concentration of the surfactant with a known Griffin's HLB value being the same as the concentration of the surfactant in the aqueous sample; adding to the standard sample a phase-changing probe consisting of a compound having a different structure than that of the surfactant with a known Griffin's HLB value to quantify an amount of the phase-changing probe required until a phase structure of the standard sample changes; performing this quantification for a plurality of surfactants with different Griffin's HLB values under the same conditions to obtain a plurality of quantitative values; and preparing a calibration curve from the quantitative values and the HLB values of the surfactants included in the standard samples that provide the quantitative values, preparing a measurement sample by adding to the aqueous sample the same oil component and optional solvent as those added to the precursor sample in the same respective amounts as those in the precursor sample; adding to the measurement sample the same phase-changing probe as the one used for the measurement of the quantitative values of the calibration curve; and quantifying an amount of the phase-changing probe required until a phase structure of the measurement sample changes, and with values of the HLB value in the calibration curve defined as liquid physical property indexes, estimating the physical property of the aqueous sample from a liquid physical property index corresponding to a quantitative value of the phase-changing probe for the measurement sample; wherein the phase-changing probe is a compound selected from polyoxyethylene alkyl ether, polyoxypropylene alkyl ether, monoalkyl monoglyceryl ether, alkyl polyglyceryl ether, polyalkyl glycoside, polyoxyalkylene alkylamine, polyoxyalkylene fatty acid ethanol amide and polyoxyethylene sorbitan alkyl ester.