adding an oil component and a phase-changing probe to the aqueous sample, and estimating the hydrophobicity of the aqueous sample from an amount of the phase-changing probe when a phase structure of the mixture changes;

wherein the phase-changing probe is a compound represented by the following general formula (1):

wherein R¹ is a hydrocarbon group with 2 or more and 9 or less carbons; R² is a hydrogen atom or a hydrocarbon group with 1 or more and 20 or less carbons; A is an alkylene group with 2 or more and 3 or less carbons; and n is a number of 0 or more and 10 or less when R² is a hydrogen atom and a number of 1 or more and 10 or less when R² is a hydrocarbon group; and

wherein the surfactant is an anionic surfactant, a cationic surfactant, an amphoteric surfactant, or a nonionic surfactant that is selected from polyoxyalkylene alkyl ether of the following formula (2), polyoxyalkylene alkylaryl ether, polyoxyalkylene alkyl thioether, polyoxyalkylene alkylaryl thioether, polyoxyalkylene alkyl ester, polyoxyalkylene sorbitan monoalkyl ester, polyoxyalkylene alkylamine, polyoxyalkylene alkyl amide, alkyl glyceryl ether, saccharide ester and fatty acid ethanol amide,

wherein R³ is a hydrocarbon group with 10 or more and 20 or less carbons; B is an alkylene group with 2 or more and 3 or less carbons; and m is a number of 1 or more and 50 or less.