Growing wildlife populations are causing serious conflicts with human, highlighting the increasing necessity for effective population management. Herbivore populations, in particular, exert a profound impact on ecosystem function, structure, and diversity. Therefore, reliable population monitoring serves as a crucial foundation for implementing appropriate management strategy. In this study, we set a total of 211 camera traps on the Boso Peninsula (southern Chiba Prefecture) and continuously monitored three herbivore species, Sika deer, Reeve's muntjac, and Japanese hare, over a four-year period. A state-spatial model was constructed using a density estimation model for unmarked populations, namely REST, to uncover temporal variation across five municipalities (Kimitsu, Futtsu, Kyonan, Minamiboso, and Tateyama). The survey was conducted from June 2018 to May 2022 (except for Tateyama, where it commenced in July 2019). Results showed that the density of the Sika deer remained consistent in all the municipalities, while Reeve's muntjacs exhibited increasing trend with a density two to six times greater than that of Sika deer. Conversely, Japanese hares exhibited a distinct pattern, experiencing a sharp decline in population density across all municipalities from 2019 to 2020. The absence of a negative correlation between muntjac and deer population density suggests limited resource competition between the two species. The study area, characterized by a mosaic of forests and rice paddies, may facilitate sympatric occurrences due to high primary production at forest edges. The dramatic decline in Japanese hare population may be likely caused by the spread of Rabbit Hemorrhagic Disease (RHD). RHD is a highly fatal infectious disease that infects species belonging to the genera Oryctolagus and Lepus. In March 2020, a year following the confirmed sharp decline in the wild Japanese hare, an outbreak of the RHD has been reported at an exhibition facility in Chiba Prefecture, Japan. It is likely that the RHD virus entered the study area through some route around mid-2019, subsequently leading to the reinfection of domestic hares. This marks the first report of RHD in wild hare population in Japan. Since the forage resources during winter among these three herbivores are overlapping, the recovery of the Japanese hare population may be affected by an increase in the population density of the two ungulate species. We need to continue to monitor and track the populations of these three species.