Recently, reduction of fish catch and poor harvest of cultured laver in the Ariake Bay have been a serious social problem in Japan. It is suspected that construction of the Isahaya Sea-Dike causes deterioration of the aquatic environment in the Ariake Bay. Although several studies on the aquatic environment have ever been carried out, the causes of the deterioration have not been understood clearly. In this study, in order to understand the role of a tidal current in the tidal exchange phenomena in the northern coastal area off Shimabara Peninsula, in situ measurements on a tidal current by Acoustic Doppler Current P rofilers (ADCPs) and on density stratification by multi-factor profilers were carried out. As a result of this study, it was observed that the phase of the tidal current near the coast is earlier than other areas. It is considered that the characteristic of the tidal current is brought about by oscillatory boundary layer. Findings of the present study imply that it is very important to carry out high-resolution field observation and numerical simulation in order to understand tidal transport well.