The present invention provides: a nucleic acid having an activity to inhibit the expression of MASP2; a pharmaceutical composition containing the nucleic acid; and a prophylactic or therapeutic agent for autoimmune diseases including APS and SLE and thrombosis occuring during hemodialysis, which contains the nucleic acid.本發明提供一種具有MASP2之表現抑制活性之核酸、含有該核酸之醫藥組合物以及含有該核酸之APS、SLE等自體免疫疾病以及血液透析時之血栓症之預防或治療藥。