Source of suspended sediment was identified from grain size distribution in two headwater basins in West Central Hokkaido, Japan. The fine grains in river bed sediment were supplied to the river water as suspended sediment in the early snowmelt season, whereas those in hillslope sediment were washed out into the rivers in the summer rainy season. The fine grains probably accumulated in the bed sediment during winter, and were stirred up into river water as the river discharge increased. Since the fine grains in the river bed sediment were gradually exhausted, further stirring up could occur only when the river discharge exceeded that season's previous maximum. The fine grains in the hillslope sediment were most likely to be stored during the dry period from May to July, only to be washed away during the summer rainy season. Thus the suspended sediment supply from the hillslope is sparse in the snowmelt season. This type of seasonal variation in suspended sediment supply is probably common to headwater basins in the cold and humid climate of West Central Hokkaido, Japan.