Optical properties of sea vater were studied in Tokyo Bay, Japan, based on the data obtained from six oceanographic station in April, 1985. The observation of surface and underwater irradiances of sea water for eight kinds of wavelength (378, 422, 481, 513, 570, 621, 653, and 677 mm) of sun light was conducted using the underwater irradiameter (Isigawa ${sharp}SR-8$). The mean attenuation coefficient of the sea water appeared to be 0.335($0.081{sim}0.862$) and the attenuation coefficient of the sea water for each wavelength appeared as 0.268 for 378nm, 0.354 for 422nm, 0.274 for 481nm, 0.256 for 513nm, 0.284 for 570nm, 0.356 for 621nm, 0.425 for 653nm, and 0.464 for 677nm. The transparency was 5.0m ($2.5{sim}6.5m$), water color was 10.2 ($8{sim}14.0$) in the study area and the sun altitude was $53.62^{circ}$ ($38.54^{circ}{sim}66.23^{circ}$). The relationship between attenuation coefficient (K) and transparency (D) was K= 2.22/D ($1.30/D{sim}3.54/D$). The rates of light penetration for eight kinds of wavelength (378, 422, 481, 513, 570, 621, 653, and 677 nm) were computed with reference to the surface light intensity each. The mean rates of light penetration in proportion to depths were $62.72\%$ ($42.23{sim}78.43\%$) in 1 m layer, $11.91\%$ ($1.34{sim}29.67\%$) in 5m layer, $2.64\%$ ($0.023{sim}8.80\%$) in 10m layer, and $0.50\%$ ($0.02{sim}3.99\%$) in 20 m layer. The rate of light penetration at the transparency layer with reference to the surface light intensity was shown as $12.51\%$ ($2.91{sim}27.25\%$).