To study the spatial structure of midlatitude sporadic E (Es) layers, the ultraviolet resonant scattering by magnesium ions (Mg+) in an Es layer was observed during the evening twilight with the Magnesium Ion Imager (MII) on the sounding rocket launched from the Uchinoura Space Center in Kagoshima, Japan. The in situ electron density measured by an onboard impedance probe showed that the Es layer was located at an altitude of 100 km during both the ascent and descent of the flight. Simultaneous observation with a ground-based ionosonde at Yamagawa identified the signature of horizontally "patchy" structures in the Es layer. The MII successfully scanned the horizontal Mg+ density perturbations in the Es layer and found that they had patchy and frontal structures. The horizontal scale and alignment of the observed frontal structure is generally consistent with a proposed theory. To our knowledge, this is the first observation of the two-dimensional horizontal structure of Mg + in an Es layer.