Successive observation of oceanographic conditions was carried out at Kure Marine Station, Hiroshima University from April, 1989 to April, 1990. Results of the observations are summerized below. 1) Vertical distribution of density factor sigma-t indicated that seawater was stratified from May to September but it was vertically well mixed from October to December. During the period from January to April, weak density gradient was observed only in the upper part of the sea. 2) Seasonal variation of seawater temperature was clear. It varied with air temperature showing some delay of the phase according to depth. Salinity decreased both in July and in September especially in the surface water affected by heavy rainfall but increased toward winter maximum. 3) Phytoplankton was generally abundant during spring through autumn and then decreased toward winter. However it decreased also in August in the surface water because of nutrient deficiency. 4) Seasonal variation of transparency was distinct showing summer minimum in July and winter maximum in January. Transparency also varied with salinity and inversely with chlorophyll a. 5) It is concluded from the results that oceanographic conditions of the station were strongly affected by the discharge of Niko River which primarily influenced salinity and nutrient supply. This then affects phytoplankton growth which was also affected by solar radiation and consequently transparency was influenced by the phytoplankton abundance.