The conservation history of the horseshoe crab Tachypleous tridentatus and its spawning ground, a designated natural monument in Kasaoka Bay in Okayama Prefecture, was investigated. Kasaoka Bay was one of the famous spawning sites of the horseshoe crab in western Japan. A large-scale land reclamation of this bay began in 1968, resulting in the disappearance of the ebb tidal flat. As one protective measure, a different habitat of this animal was additionally designated a natural monument. However, the population of the horseshoe crab has become endangered. Insufficient consideration of environmental capacities and physical factors is a critical point. This history provides useful information for reconsidering the present and future environmental conservation activities for rare animal species.