PROBLEM TO BE SOLVED: To provide magnetic particles which enhance immunoreactivity without reducing its particle diameter.SOLUTION: The magnetic particles contain superparamagnetic metal oxide particles (C) having an average particle diameter of 1-15 nm in a non-magnetic metal oxide (D). The superparamagnetic metal oxide particles (C) are contained in an amount of 60-95 wt.% based on the total weight of the superparamagnetic metal oxide particles (C) and the non-magnetic metal oxide (D). The magnetic particles have an average particle diameter of 1-5 &mum, a shape factor SF1 of 101-200, and a shape factor SF2 of 110-220. Preferably, the non-magnetic metal oxide (D) is at least one selected from the group consisting of silicon oxide, titanium oxide, zirconium oxide, aluminum oxide, calcium oxide, yttrium oxide, and tantalum oxide, and the superparamagnetic metal oxide particles (C) are made of iron oxide.
