The present invention provides a complex, which comprises a cationic liposome comprising 2-O-(2-diethylaminoethyl)carbamoyl-1,3-O-dioleoylglycerol and a phospholipid as main ingredients, and an oligo nucleic acid which is carried on the liposome. Also, the present invention provides a pharmaceutical composition containing the complex to be used for treating or preventing diseases caused by a target molecule of the oligo nucleic acid (target DNA, target RNA, target protein) since it is possible to administer the complex in vivo and to put the complex into practical use as a medicament because the complex can exhibit pharmacological efficacy in vivo.