An object of the present invention is to provide surface-modified ferromagnetic iron oxide particles suitable for electromagnetic-induction cancer ablation, which have excellent heat generation properties and also have excellent dispersion stability in a medium for injection. The surface-modified iron oxide particles for cancer ablation of the present invention as a means for achieving the object are characterized in that a block copolymer of polyethylene glycol and polystyrene having a phosphorous acid group in the side chain is bound to the surface of ferromagnetic iron oxide particles having a plate-like shape with a length of 20 to 200 nm and a length-to-thickness ratio of 1.5 to 30 and having magnetic properties such that the coercivity is 30 to 300 Oe, the saturation magnetization is 20 to 80 emu/g, and the squareness ratio of the magnetic hysteresis loop is 0.20 to 0.50.
