The present invention provides a nucleic acid synthesis method capable of continuously carrying out an extension reaction and a single molecule sequencing method capable of obtaining base information accurately at high speed. A method for synthesizing a nucleic acid, including the steps of: forming a complex of a target nucleic acid hybridized to a primer oligonucleotide and a DNA polymerase β; allowing the DNA polymerase β to incorporate a fluorescently-labeled dNTP so that the fluorescently-labeled dNTP is bound to the 3′ end of the primer oligonucleotide; and allowing the DNA polymerase β to continuously incorporate fluorescently-labeled dNTP to extend a nucleic acid complementary to the target nucleic acid from the 3′ end of the bound fluorescently-labeled dNTP. A method for sequencing a single nucleic acid molecule, including the steps of said method for synthesizing a nucleic acid, wherein fluorescence emitted from each of the fluorescently-labeled dNTP incorporated into the DNA polymerase β is sequentially detected to carry out the sequencing of the target nucleic acid.