Field measurements on topography and groundwater level were conducted on the Nijigahama and Matsuzaki coasts in Japan, where the gravity drainage system for beach stability was installed under the surfaces of the beaches. The system prevented the seaward loss of sands although it was not applicable to beaches eroded by the imbalance of longshore sediment transport rate. It did not harm pine trees landward of beaches. Points of notice for the application of the system were also presented.