To obtain the subject particle having an enlarged specific surface area, capable of exhibiting a high effect as a microorganism flocculating agent by bonding an organic group containing a pyridinium base to a fine inorganic particle.

This inorganic particle comprises an organic group containing pyridinium base on the surface. The inorganic particle is preferably selected from a metal, a metal oxide, a metal nitride, a metal carbide and a metal sulfide and the diameter of the inorganic particle is preferably ≤5 μm, preferably ≤1 μm. The organic group is preferably a polymer. The bonding of the organic group containing the pyridinium base to the inorganic particle can be carried out by formulating an organic group precursor containing the pyridinium base with the inorganic particle in a proper solvent or by a reaction such as hydrolysis or condensation. Preferably, a substance having selective bondability (preferably an antigen or an antibody) is bonded to the surface of the inorganic particle.