A method for operating magnetic body particles that is for fixing a target substance in a liquid sample to the surface of the magnetic body particles; and a device for operating magnetic body particles that is used for said method. The magnetic body particles are particles to which a target substance can be fixed selectively. In this method, in a state where a liquid sample, the magnetic body particles, and a magnetic solid body that has a larger particle diameter than that of the magnetic body particle are made to coexist in a container, the magnetic body particles are moved together with the magnetic solid body within the liquid sample by operating a magnetic field from outside the container. By this operation, the target substance can be fixed selectively to the surface of the magnetic body particles.