PURPOSE: To prepare Fomes Japonicus preserved in vinegar wherein protein and a linked substance of protein and saccharide are stabilized with an acid, by immersing Fomes Japonicus in a brewed vinegar adjusted to a specific acidity, and, if necessary, heating it.

CONSTITUTION: Small pieces of Fomes Japonicus clearly produced are immersed in purified water obtained by removing chlorine, etc. from ciry water without washing the pieces with water, and the water-soluble active ingredients of Fomes Japonicus are extracted with water at normal temperature. A concentrated brewed vinegar is added to the mixture of Fomes Japonicus and the water- soluble active ingredients and the solution is adjusted to 4W6wt% acidity. The solution is heated, kept at the temperature, and active ingredients eluted in the acidic solution are extracted. The mixture of Fomes Japonicus and brewed vinegar after heating is bottled, and, if necessary, the mixture and the bottle are sterilized under heating, and cooled to give a product.