As a means for solving the problem of providing a nucleic acid complex that rarely undergoes the decomposition at a DNA-RNA joining site therein even in vivo, a nucleic acid complex is provided, which is produced by linking 3'-position of a 3'-terminal deoxyribonucleotide residue in single-stranded DNA to 5'-position of a 5'-terminal ribonucleotide residue in one ribonucleotide strand in double-stranded RNA, wherein a hydroxyl group located at 2'-position of the 5'-terminal nucleotide in the ribonucleotide strand that is bound to the single-stranded DNA is substituted by an alkoxy group or a halogen group and/or a phosphoric diester group between 3'-position of the first ribonucleotide that is bound to the single-stranded DNA and 5'-position of a ribonucleotide adjacent to the first ribonucleotide is substituted by any one selected from a phosphorothioate group, a dithiophosphoric diester group and a trithiophosphoric diester group.提供一種解決即便於生物體內，亦不會在DNA與RNA之連結部被分解之核酸複合體這一課題之手段，提供如下之核酸複合體，該核酸複合體係由單鏈DNA之3'側末端之去氧核糖核苷酸殘基之3'位與雙鏈RNA之一個核糖核苷酸鏈之5'側末端之核糖核苷酸殘基之5'位鍵結而成的核酸複合體，且該與單鏈DNA鍵結之核糖核苷酸鏈之5'側末端核苷酸中的2'位之羥基係經烷氧基或鹵基取代，及/或與該單鏈DNA鍵結之最初之核糖核苷酸之3'位及與其鄰接之核糖核苷酸之5'位之間的磷酸二酯基係經硫代磷酸酯基、二硫代磷酸二酯基及三硫代磷酸二酯基之任一者取代。
