A sensitive optical magnetic field sensor was experimentally demonstrated using Ni-subwavelength grating (SWG) combined with a SiO<sub>2</sub>/Ag plasmonic structure. We fabricated the Ni-SWG structure on the Ag/SiO<sub>2</sub> structure using electron beam lithography and a liftoff process. As a result, a dip in the reflection spectra with normal incidence was obtained at a wavelength of 530 nm. The reflectivity at the dip position significantly decreased with the intensity of the magnetic field applied to the structure. When a magnetic field of 43 mT was applied, the change in reflection reached approximately 4% of that without magnetic field. The experimental results indicate that our sensor achieves millitesla order of sensitivity for the magnetic field. The electromagnetic field distribution around the Ni-SWG/SiO<sub>2</sub>/Ag calculated using the finite-difference time-domain method clarified the reason for the high sensitivity of our sensor.