To develop new therapies for chronic diseases where conventional treatments fail or are less effective. The present invention provides a pharmaceutical composition for treating and / or preventing inflammatory chronic diseases, which is used in combination with a biologic or the like that inhibits tissue infiltration of leukocytes. The pharmaceutical composition of the present invention is effective for siRNA that suppresses the expression of the CHST15 gene, comprising a structure in which an RNA comprising the nucleotide sequence set forth in SEQ ID NO: 1 is hybridized with an RNA complementary to the RNA. Contains as an ingredient.従来療法では不奏効又は効果が低い慢性疾患に対する新規療法を開発すること。 本発明は、白血球の組織浸潤を阻害する生物製剤等と併用する、炎症性慢性疾患の治療及び／又は予防用医薬組成物を提供する。本発明の医薬組成物は、配列番号：１に記載の塩基配列を含むＲＮＡと、該ＲＮＡと相補的なＲＮＡとがハイブリダイズした構造を含む、ＣＨＳＴ１５遺伝子の発現を抑制するｓｉＲＮＡ等をを有効成分として含む。
