This invention relates to an allergen inactivating agent containing a polysaccharide derivative as its effective component, wherein said polysaccharide derivative has a cellulose ether or a starch ether as its backbone, and some or all of hydrogen atoms in the hydroxy group of the polysaccharide derivative are substituted by a group represented by the following general formula (1): -E<；1>；-(OA)n-E<；2>；-R wherein E<；1>； represents an alkylene containing 1 to 6 carbon atoms optionally substituted with hydroxy group or oxo group； n represents a number of 0 to 50； A independently represents an alkylene containing 1 to 6 carbon atoms, the number of A being n； E<；2>； represents ether bond or oxycarbonyl group； R represents an alkyl group containing 4 to 30 carbon atoms optionally substituted with hydroxy group, a sulfoalkyl group containing 1 to 5 carbon atoms optionally substituted with hydroxy group, or a salt thereof. <；??>；By using such allergen inactivating agent, house dust and other environmental allergens can be inactivated with no adverse effects on the human body and without causing problems such as color development.
