To provide an antiallergenic composition capable of efficiently deactivating allergens derived from house dust, mite, pollen, etc. in the environment even under dry conditions, and to provide an antiallergenic filter medium or sheet obtained by processing the composition.

The antiallergenic composition comprises an allergen deactivator consisting of a plant belonging to the genus Olea europea or Ligustrum obtusifolium, Oleaceae family or its extract and a hygroscopic substance such as an inorganic salt. The filter medium or sheet obtained by processing this composition is provided. A method for deactivating allergens involving using this filter medium or sheet is also provided.