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Magnetic nanoparticles and their detection systems are widely applied to the diverse biomedical applications, and the more accurate detection techniques have been explored. Here, we propose the cavity-shaped magnet that enables the application of the highly sensitive magnetic sensor for the enhancement of the detection ability. The numerical simulations based on the finite element method revealed the wide area of the magnetic null point (B ∼ 0 and ∇B ∼ 0) of the cavity-shaped magnet. The strong gradient of magnetic fields of ∇BZ ∼ 160 mT/mm in the conventional column-shaped magnet reduces to approximately zero and the wider magnetic null area is obtained in the cavity-shaped magnet. The magnetic null area of the column- and cavity-shaped magnet are ∼0.1 × 0.05 mm2 and ∼0.25 × 0.6 mm2, respectively, indicating that the area expands to approximately 30 times larger. This scale-up of the magnetic null area in the proposed cavity-shaped magnet suggests the application of the highly sensitive magnetic sensor, thereby would be offering the more accurate magnetic nanoparticle detection for biomedical applications.