An object of the present invention is to develop a novel treatment method for chronic diseases for which conventional treatment methods are either ineffective or for which efficacy is low. The present invention provides a pharmaceutical composition for the treatment and/or prevention of an inflammatory chronic disease that is used in combination with a biological preparation that inhibits leukocyte tissue invasion. The pharmaceutical composition of the present invention contains as an active ingredient thereof siRNA suppressing the expression of CHST15 gene that contains a structure formed by the hybridization of RNA containing the base sequence represented by SEQ ID NO: 1 with RNA complementary thereto.
