Medicinal composition for treating and preventing chronic disease

The present invention addresses the problem of developing a novel therapy for a chronic disease, against which conventional therapies show no or little efficacy, and provides a medicinal composition for treating and/or preventing an inflammatory chronic disease, said medicinal composition being to be used together with a biological preparation etc. capable of inhibiting tissue invasion by leukocytes. The medicinal composition according to the present invention comprises, as an active ingredient, siRNA suppressing the expression of CHST15 gene, etc., said siRNA containing a structure formed by the hybridization of RNA containing the base sequence represented by SEQ ID NO: 1 with RNA complementary to the aforesaid RNA.