The present invention relates to an oligonucleotide having a novel structure and a method of synthesizing nucleic acid by using the same as a primer. This oligonucleotide is provided at the 5′-side of the primer with a nucleotide sequence substantially the same as a region synthesized with this primer as the origin of synthesis. The present invention realizes synthesis of nucleic acid based on an isothermal reaction with a simple constitution of reagents. Further, the present invention provides a method of synthesizing highly specific nucleic acid on the basis of this method of synthesizing nucleic acid.