The present invention relates to a magnetic reagent, a magnetic reagent kit, a method for treating magnetic carriers, and a treatment device therefor, with an object of quickly and efficiently treating magnetic carriers of a micro particle diameter including nanosized magnetic carriers.The magnetic reagent or magnetic reagent kit includes: a plurality of magnetic carriers which can be magnetized by being exposed to a magnetic field, can be bonded to a predetermined chemical substance or living organism in a liquid, and that have a particle diameter that enables them to be suspended in the liquid and a plurality of treatment promoting magnetic particles which can be magnetized by being exposed to a magnetic field and thus can have the magnetic carriers adsorbed on the surface thereof, are formed so that they can be moved within the liquid by movement of the liquid or by a magnetic field, and promote a treatment for capturing or re-suspension of the magnetic carriers.
