Disclosed are: an oligonucleotide useful for the inhibition of the production of IL-17, the inhibition of the proliferation of a CD4-positive T cell, the amelioration of an inflammation induced by IL-17 or the like； and an NR4A2 gene expression inhibitor, an IL-17 gene expression inhibitor, a CD4-positive T cell proliferation inhibitor or a pharmaceutical composition comprising the oligonucleotide as an active ingredient. Specifically disclosed is siRNA which can inhibit the expression of NR4A2 gene, such as an oligonucleotide having a nucleotide sequence depicted in any one of SEQ ID NOs:1 to 3. The siRNA is useful for the inhibition of the expression of NR4A2 gene, the inhibition of the expression of IL-17 gene, the inhibition of the proliferation of a CD4-positive T cell, the amelioration of an inflammation induced by IL-17 or the like.
