We report the first detection of an hourglass magnetic field aligned with a well-defined outflow-rotation system in a high-mass star-forming region . The observations were performed with Submillimeter Array toward G240.31+0.07 , which harbors a massive , flattened , and fragmenting molecular cloud core and a wide-angle bipolar outflow . The polarized dust emission at 0.88 mm reveals a clear hourglass-shaped magnetic field aligned within 20 ^ { \circ } of the outflow axis . Maps of high-density tracing spectral lines , e.g. , H ^ { 13 } CO ^ { + } ( 4–3 ) , show that the core is rotating about its minor axis , which is also aligned with the magnetic field axis . Therefore , both the magnetic field and kinematic properties observed in this region are surprisingly consistent with the theoretical predictions of the classic paradigm of isolated low-mass star formation . The strength of the magnetic field in the plane of sky is estimated to be \sim 1.1 mG , resulting in a mass-to-magnetic flux ratio of 1.4 times the critical value and a turbulent to ordered magnetic energy ratio of 0.4 . We also find that the specific angular momentum almost linearly decreases from r \sim 0.6 pc to 0.03 pc scales , which is most likely attributed to magnetic braking .