To provide a method and a preservative for improving the preservability of processed foods retaining microbial proliferation inhibitory action for a long period without impairing their flavor by using a highly safe natural extract component.

An extract liquor is prepared by mixing extract components from the leaves, pericarps or seeds of Betula tauschii, Prunus padus L. or citrus with a polyhydric alcohol so that the content of the extract components in the liquor is 40-60 wt.%, the obtained extract liquor is mixed, at an appropriate percentage, with an aqueous ethanol solution having ≤40 wt.% practical ethanol concentration to be diluted, mixed with lactic acid or butyric acid of 3-8% weight, and mixed with chitosan of 3-8% weight to dissolve and, thereby, forms the preservative. A processed food is mixed, impregnated or sprayed with the preservative to improve preservability thereof.