Abstract: A method for measuring fibroblast growth factor-23 (hereinafter, indicated as FGF-23) in a sample, which comprises the steps of: (1) reacting in an aqueous medium, a sample and a first antibody or an antibody fragment thereof which binds to FGF-23; (2) reacting in an aqueous medium, a second antibody or an antibody fragment thereof which binds to FGF-23 with Immunocomplex 1 produced in the aforementioned step (1) to produce Immunocomplex 2 consisting of the first antibody or the antibody fragment thereof which binds to FGF-23, FGF-23, and the second antibody or the antibody fragment thereof which binds to FGF-23; and (3) measuring Immunocomplex 2 produced in the aforementioned step (2); wherein the reaction of step (1) and/or step (2) is carried out in an aqueous medium comprising a surfactant such as a polyoxyethylene alkyl ether.