During the period from 1987 to 1994, the harvests of Japanese short-necked clam, Ruditapes philippinarum, were kept between 98 to 160 tons each year on the estuary of the Banjyo River in Saiki Bay. However the harvests sharply decreased to 14 tons in 1995. We, therefore, examined the sexual maturity, recruitment, growth of the clam and environmental factors in the ground from 2001 to 2004 to find out the clam population factors. In environmental factors, water temperature ranged 12.1-30.0℃ on the bottom of the water, 9.7-28.2℃ on the tidal flats, and salinity ranged 6.8-10.6psu on the tidal flats at low water. At various ground levels, emergence of tidal flats was about 3-7 hours during spring tides and river discharge in normal year was lower, below 20m^3/sec. The population maturity rate increased from the middle April to the middle November and reaches its peak from September to October. Therefore, the main spawning season for the clams seems to be in autumn. After settlement, the juvenile clams with 5-8 mm in the average shell length appeared from July to September in the ground. The origin of this recruitement was estimated in the previous autumn fisheries population. The length frequency analysis shows that the clams grow up to 9-10mm in shell length for age of 1 year, and 18-20mm in shell length for age of 2 years. This growth rate is lower than those in the Ariake Sea, Maizuru Bay and Tokyo Bay. We suggest that the lower growth rate of the clam results from low salinity, long emergence hours and low river discharge.