The residual electron density distribution in magnesium aluminum oxide (natural spinel) has been studied with single-crystal X-ray diffraction methods. Structure refinement incorporating an experimental scattering factor refinement technique showed a positive residual with 0.7-1.0e/Angstrom(3) height at the center of the Al4O4 cuboid in the structure. [200 equivalent-averaged reflexions, R values; harmonic refinement : 0.0127, anharmonic (Gram-Charlier series expansion up to the 6th-rank tensors) : 0.0109]