PURPOSE: To simply perform immunoassay consistently in a flow system, by reacting a liquid to be examined, an antibody to an antigen being a measuring object and a labelled antigen and subsequently passing the reaction mixture through a filter means excluding substances with a definite MW or less.

CONSTITUTION: A buffer solution to which an antibody 1, a sample 2 and a labelled antigen 3 are added passes through a thermostatic tank 6 while rotates spirally to be adjusted to constant temp. under stirring and immunological reaction is promoted. Next, the liquid 4 passes through the hollow fiber 5 contacted with an external liquid 7. At this time, the liquid 4 rotates spirally and, at the same time, the external liquid 7 is stirred by stirrers 9, 10. When the liquid 4 passes through the hollow fiber 5 excluding a substance with a definite MW or less, the antigen 3 not bonded to the antibody 1 is promptly eluted to the outside of the hollow fiber 5. Therefore, the process of immobilizing the antibody 1 is omitted and, further, washing operation at the time of bound/free separation can be eliminated and, therefore, immunoassay is simply performed in a consistent flow system.