Abstract: The present invention addresses the problem of making it possible, in a simple immunoassay, to prevent false-positive reactions and reduce measurement value errors so as to accurately detect fFN by suppressing a cross-reaction with plasma FN that is an antigenic component similar to fFN to reduce the detection of plasma FN. When an anti-fFN antibody reacts with fFN in the presence of a specific surfactant according to the present invention, it is possible to accurately detect fFN through the suppression of the cross-reaction between the antibody and FN, even when other FNs such as plasma FN are also present. The specific surfactant is represented by general formula (1): R—O—(CH2CH2O)mH??(1) In the formula, m is 4-30, and R represents an alkyl group.