Sediment budgets on an intertidal flat at the mouth of the Shirakawa River in Kumamoto, Japan, where the averaged offshore wave height is 0.33 m and the mean spring tidal range is 4.5 m, were investigated using bathymetry, river discharge and wave data. The investigated period from 1976 to 2002 was divided into four short periods according to the dates when surveys were conducted, and sediment budget quantities during the short periods including the volume changes, the amount of sediment supply from the river and the sediment transport rate due to waves and tidal currents were estimated. The estimated sediment budget quantities with large sediment supply from the river significantly differed from those with less sediment supply, and the development and decay of the flat strongly depended on the amount of sediments discharged from the river. With large supply from the river of 6.8 x 10**5 m3/yr of sediment, which was caused by major floods, the flat had accumulation and the accumulation rate was 3.5 x 10**5 m3/yr. on the other hand, the flat was eroded at a rate of 2.5 x 10**5 m3/yr with little supply from the river of 0.8 x 10**5 m3/yr of sediment.