A method using a membrane-concentration method and a freeze-concentration method includes: a membrane-concentration step in which a fluid to be treated is cooled and a membrane-concentrated fluid is prepared by membrane-concentrating the solid content concentration thereof by more than 1.5 times; an ice crystal generation step in which said membrane-concentrated fluid is cooled, ice crystals of said membrane-concentrated fluid are generated in said membrane-concentrated fluid, and a mixed fluid to be treated is formed wherein said mixed fluid to be treated is comprised of said ice crystals and a concentrated fluid to be treated produced from said membrane-concentrated fluid by generating said ice crystals in said membrane-concentrated fluid thereby said membrane-concentrated fluid is concentrated; and an ice crystal separation step in which said mixed fluid is separated into said concentrated fluid to be treated and said ice crystals, and said separated concentrated fluid to be treated is retrieved.