The present invention relates to an RNAi-inducing nucleic acid molecule having a new structure and the use thereof, and more particularly to a novel nucleic acid molecule having a structure comprising a first strand, which is 24-121 nt in length and comprises a region complementary to a target nucleic acid, and a second strand which is 13-21 nt in length and has a region that binds complementarily to the region of the first strand, which is complementary to the target nucleic acid, so that the nucleic acid molecule inhibits the expression of a target gene with increased efficiency, and to a method of inhibiting the expression of a target gene using the nucleic acid molecule. The nucleic acid molecule structure of the present invention increases the efficiency with which the nucleic acid molecule inhibits the target gene. Alternatively, the nucleic acid molecule of the present invention can either increase the ability of the siRNA to bind to the target gene or cause synergistic cleavage, by introduction of antisense DNA, antisense RNA, ribozyme or DNAzyme, thereby increasing the efficiency with which the nucleic acid molecule inhibits the target gene. In addition, when the nucleic acid molecule according to the present invention is used, the efficiency with which the target gene is inhibited can be maintained for an extended period of time. Accordingly, the RNAi-inducing nucleic acid molecule of the present invention can be effectively used for the treatment of cancer or viral infection in place of conventional siRNA molecules.本発明は、新規構造のＲＮＡｉを誘導する核酸分子及びその用途に関し、より詳細には標的核酸（target nucleic acid）と相補的な一部領域を含む２４～１２１ｎｔの長さの第１鎖と、前記第１鎖の標的核酸と相補的な一部領域と相補的結合を形成する領域を持つ１３～２１ｎｔの長さの第２鎖とを含む構造を持つようにして、標的遺伝子の発現抑制効率を増加させた新しい構造の核酸分子及びこれを利用した標的遺伝子の発現抑制方法に関する。本発明に係る核酸分子構造は、遺伝子抑制効率を増加させるか、またはａｎｔｉｓｅｎｓｅ ＤＮＡ、ａｎｔｉｓｅｎｓｅ ＲＮＡ、リボザイム、ＤＮＡザイムの導入によって、ｓｉＲＮＡと共に同一標的遺伝子に対する結合力向上、または相乗的開裂が起きる効果があって、標的遺伝子抑制効率を増加させることができる。さらに、本発明に係る核酸分子を用いることによって、遺伝子抑制効率の持続期間を延ばすことができる。従って、本発明に係るＲＮＡｉを誘導する核酸分子は、従来のｓｉＲＮＡ分子の代わりに、
