This paper describes the interpretation of landslide behavior and verification of the effectiveness of countermeasure works based on the results of long-term field observations at a large-scale cutting site. The site has been subjected to various deformations since its construction due to its unique geological conditions, and has been monitored extensively by GPS surface displacement gauges, borehole inclinometers, anchor load cells, and water level gauges as an important monitoring site even after it was put into service. In the sixth year after the site was opened to public use, we assumed underground slip surfaces based on an interpretation of the observed data, constructed additional countermeasures, and verified the effectiveness of the countermeasures through continuous field observation of intermittent landslide behavior observed in several areas of the slope. As a result, it was confirmed that the displacement had not been settled even five years after the construction of the additional countermeasures and that a new slip surface had emerged, indicating the necessity of improving field observation techniques and data interpretation as well as continuous monitoring of this site.