A two-core optical fiber magnetic field sensor is configured from at least a light incidence/emission unit; a lens; a magnetic garnet; and a reflector, wherein the lens and the magnetic garnet are disposed between the light incidence/emission end of the light incidence/emission unit and the reflector; a light beam is emitted from one optical fiber; the light beam is reflected by the reflector after being transmitted through the lens and the magnetic garnet; the light beam is transmitted again through the magnetic garnet and the lens after the reflection; and incident on the other optical fiber, the light beam is emitted again from the other optical fiber, and reflected by the reflector after being transmitted through the lens and the magnetic garnet; and the light beam is transmitted again through the magnetic garnet and the lens after the reflection and incident again on the one optical fiber.