Disclosed is a composition for treating dementia including shRNA to inhibit expression of S100a9. More particularly, the present disclosure describes a composition for prevention or treatment of dementia which includes shRNA having a nucleotide sequence defined by SEQ. ID No. 1 or 2 or a mixture thereof wherein the nucleotide sequence is complementarily bonded to mRNA of S100a9 in order to inhibit expression of S100a9, as well as a method for prevention or treatment of dementia, including administering the foregoing shRNA into a mammalian cell including a human cell or in vitro established mammalian cell-line, in order to inhibit expression of S100a9 protein.