The optical properties of sea water in Hiroshima Bay were investigated. Observed attenuation coefficient in water column ranged between from 0.11 m^&lt-1&gt to 0.46 m^&lt-1&gt, and were larger than the values estimated from in situ chlorophyll a concentration using reported equations. The factor in the Poole and Atkins equation for estimating the attenuation coefficient form Secchi disk depth was estimated to be 1.6. The ratio of the euphotic depth to Secchi disk depth was calculated to be 2.8 for Hiroshima Bay water.