In order to investigate the present state of marine environment in Niigata Prefecture, and to investigate the behavior of rare earth elements (REEs) in the ocean, REEs in many kinds of seaweed samples taken on the coast in Niigata Prefecture were determined by inductively coupled plasma mass spectrometry (ICP-MS) .<BR>Seaweed samples were taken at 10 spots on the coast in Niigata Prefecture including Sado Island on August, 2000. After decomposing the samples, REEs were separated by solvent extraction method.<BR>The following results are obtained. (1) Even in the same phylum (e.g., green algae, brown algae, or red algae), concentrations of REEs vary widely depending on the species of seaweed. (2) A large difference was found between REE patterns of the seaweed samples taken on the coast in Niigata Mainland and those on the coast in Sado Island. It shows that the surrounding marine environment seems to be an important factor for determining REE pattern as well as the species of seaweed. (3) As to REE pattern of some seaweed samples taken on the coast in Niigata Prefecture, Yb-anomaly was found as well as Ce-, and Eu-anomaly. (4) REE pattern of seaweed samples may be an useful tracer of investigating the surrounding marine environments.