This paper reports a novel resonant magnetic sensor using a magnetic gradient field formed by a magnetic concentrator consisted of asymmetric and facing two permalloy plates. A Si cantilever with a size of 5×15×200 μm3 is placed between the gap of the magnetic concentrator, and a 10μm-diameter magnetic particle is placed on the tip of the cantilever. The resonant frequency changes as the magnetic force is impinged on the magnetic particle by the magnetic field gradient formed by the magnetic concentrator. The extremal magnetic field can be measured from the resonant frequency change. The resonant magnetic sensor is fabricated by conventional microfabrication techniques, and the magnetic force-depended resonant frequency change is evaluated under the self-oscillation condition. The experimental magnetic resolution was estimated to be 5.1×10-7 T considering the experimental noise level (3.29 ppm). By downsizing the cantilever and optimizing the permalloy concentrator, this resonant magnetic sensor has a feasibility to achieve a magnetic field resolution of 1.7 ×10-14 T.