Daily measurement of chlorophyll-a as food index for oyster culture in Hakata Bay were taken by fluorescence method with chlorophyll meter from 2007 to 2009. High values of chlorophyll-a were coincident with observed red tides, with a total mean of 2.8 (μgl^{-1}). The 31 day running means of chlorophyll-a and other meteorological factors showed that several seasonal maxima of chlorophyll-a were occurred from spring to autumn, when water temperature, precipitation and solar irradiance exceeded 20 (℃), 10 (mm) and 20 (MJｍ^{-2}), respectively. Seasonal change in Apparent Relative Increase Rate (ARIR) of phytoplankton estimated from the measured chlorophyll-a coincided with that of precipitation. More positive correlation was found between the 7 day running means of ARIR with delay of 3 days and those of precipitation. Since the oyster culture rafts near the measurement station of chlorophyll-a were located in the west side of the bay-mouth, fluxes from open sea Genkai Nada could have great influence on oyster production. So, oysters in this area might owe most of their foods to the primary production from both the outside and the innermost of Hakata Bay where nutrients supplied by precipitation could accelerate the phytoplankton bloom to produce organic food materials.