We show presence of a mini-magnetosphere above the Reiner Gamma magnetic anomaly (RGA) region in the solar wind, using Lunar Prospector magnetometer (MAG) measurement data. RGA is one of the strongest magnetic anomalies on the Moon. Two magnetic anomalies are found from six MAG datasets at 17--40 km altitudes in the lunar wake or the geomagnetic tail lobe and are well explained by a two-dipole model. When RGA was exposed to the solar wind plasma, two MAG datasets were obtained at 27--29 km altitudes. Although the magnetic anomalies survived against the plasma pressure, they were heavily distorted in comparison with the magnetic field of the two-dipole model. Flow directions and dynamic pressures of the solar wind plasma at those periods indicate that the distortions were caused by forming a mini-magnetosphere over the RGA region in the solar wind.