Many plant and animal species that inhabit grassland in Japan are endangered because of habitat loss over the last century. Here, we mapped the historical distribution of grassland in the Chugoku region of western Japan using old editions of topographic maps (scale 1:50,000) and compared the historical distributions of grassland with the current distribution using digitized vegetation maps (scales 1:25,000 and 1:50,000). The area of grassland in the 1900s and 1950s was estimated to be 4602 km² (14.8% of the total area analyzed) and 3007 km² (9.5%), respectively. The current grassland area was estimated to be 217 km² (0.7%). Our results indicate that the area of grassland in the Chugoku region began to decrease during the first half of the twentieth century and sharply decreased during the second half of the twentieth century. Additionally, we evaluated the contribution of natural parks to the long-term conservation of grassland. The area of grassland maintained over a century was estimated to be 36.0 km², of which 23.4 km² was located in natural parks. Most of the area of grassland maintained over a century was located in Akiyoshidai Quasi-National Park (10.1 km²) and Daisen-Oki National Park (6.1 km²). Our results show that natural parks play an important role in the long-term conservation of grassland.