A method and apparatus for separating solid supports formed of magnetic particles on which immunocomplexes are bound, from a liquid phase includes a reaction container containing a fluid in which magnetic particles having immunocomplex bound thereon are suspended. The reaction container is positioned at a flock forming station, where the fluid is subjected to an alternate current magnetic field having a temporally varying intensity by an electromagnetic. The magnetic particles are flocculated with each other due to the application of such varying magnetic field to form a multiplicity of flocks. A stronger magnetic field then is applied to the resulting contents in the reaction container by a permanent magnet. The flocks are magnetically fixed on the inner wall surface of the reaction container by the action of the stronger magnetic field. While the magnetic particles are being fixed on the inner wall surface, a nozzle is inserted into the reaction container to suck the unnecessary liquid phase and remove it from the reaction container.