A magnetic sensor is provided that measures a magnetic field. The sensor includes a first gas, a probe light source which causes first circular polarized light to be incident on the first gas, a second gas arranged on an optical path of a second circular polarized light, an AC magnetic field generator which generates an AC magnetic field and generates magnetic resonance, a bias magnetic field generator which generates bias magnetic fields with different intensities for the first gas and the second gas and differentiates the optical transmittance of the first circular polarized light in the first gas from the optical transmittance of the second circular polarized light in the second gas, and a detector which detects a light amount of the first circular polarized light and a third circular polarized light.