PURPOSE: Microorganism including fine magnetic particles are treated with cell-wall digesting enzymes, lipase and protease to separate the magnetic particles of high purity in high yield in a short time.

CONSTITUTION: A solution containing aquatic bacteria containing the magnetic particles in their cells are preferably pressurized and the pressure is reduced immediately after the pressurization to break the cell walls. Then, cell-wall digesting enzymes and lipase are simultaneously or subsequently brought into contact with the microorganisms, and protease is allowed to act on the product. Finally, the reaction mixture is subjected to solid-liquid separation to obtain the objective fine magnetic particles.