A quality-improving agent for processed cereal foods which comprises water-soluble acidic polysaccharides originating in potato. Using this quality-improving agent, efflux of starch from a processed cereal food can be prevented and loss of gloss can be relieved. Further, self-stickiness of the food can be lessened and thus disintegration properties of the food are improved. Furthermore, hot water in which the food is to be reconstituted can be prevented from clouding. That is, the qualities of the processed cereal food can be improved.