We constructed the apparatus to observe the Freedericksz transition of liquid crystal in contact with water. The Freedericksz transition is a distortion of nematic liquid crystals (LCs) induced by external fields. In the present system, sweeping homogeneous magnetic field was applied to the sample, and the distortion of the LC was visualized with a polarized light microscope with the crossed Nichols configuration. The anchoring energy (W-AQ/LC) at the aqueous phase/LC interface was measured in the presence of surfactant from the threshold magnetic field of the Freedericksz transition. We studied two cationic surfactants: dodecyltrimethylammonium bromide and tetradecyltrimethylammonium bromide. A nematic LC, 4-cyano-4'-pentylbiphenyl (5CB), was examined, which was confined in a copper grid on an octadecyltrichlorosilane-treated microscope glass plate. Measured W-AQ/LC were reproducible and showed consistence with the reported region for the water/LC interface. Interfacial excess of surfactants was also measured by the pendant drop method, and the relationship between the obtained W-AQ/LC and the interfacial excess was investigated. Experiments showed that an increase in the anchoring energy depends on the surfactant and its interfacial excess. The region of the interfacial coverage, at which W-AQ/LC increases, varied with the chain length of the surfactant. The measurement of the anchoring energy will provide new fundamental information on aqueous phase/LC interface.