The downward spectral irradiance and upward spectral radiance, and chlorophyll a concentration of surface water were determined in the North Pacific Ocean: Bering Sea, Gulf of Alaska, Central and Equatorial Pacific, Kuroshio region, Yellow Sea, Japan Sea, Tokyo Bay, and Ise Bay. These areas included Case I and Case II water. The optical characteristics in the study area were based on measurements of underwater spectral irradiance. The strong correlation between the attenuation coefficient at 490 nm, K490, and other at wavelengths were observed. The relation between chlorophyll concentration and K490 showed a good correlation at eh coastal and bay areas except a the stations near river mouths. These relations suggested that suspended and dissolved matter were largely of biogenous origin such as fragmented and decomposed phytoplankton. In the clear ocean, at low chlorophyll concentrations, the data points were widely scattered, implying that K490 is sensitive to changes in suspended particles.