The present invention aims to provide a novel nucleic acid capable of suppressing expression of β2GPI, as well as a pharmaceutical composition for the prophylaxis or treatment of diseases associated with β2GPI expression.The present invention solves the above-mentioned problem by providing an antisense oligonucleotide having a β2GPI expression suppressive activity, a pharmaceutical composition containing the antisense oligonucleotide, and a prophylactic or therapeutic drug for autoimmune diseases including APS, SLE and the like, and thrombosis in hemodialysis that contains the antisense oligonucleotide and suppresses β2GPI expression.
