Along Wakimisaki beach, in the southwestern part of Nagasaki peninsula, beachrock developed in the intertidal zone, and is designated a natural monument by Nagasaki prefecture. The beachrock consists of sand and gravels cemented by carbonates in a muddy matrix, and is a conglomerate. The beachrock can be divided into five formations from its strike, dip, and location. Eight sample materials collected from the beachrock were radiocarbon dated. Calibrated radiocarbon ages suggest the beachrock formed between ca. 5,700 cal BP, and ca. 600 cal BP. There is no geomorphic and geologic evidence to suggest tectonic movements on the Wakimisaki coast. This means sea-level at the Wakimisaki coast has been stable since ca. 5,700 cal BP.