A
nucleic acid synthesis method enabling a reaction in a fluid (flow) with a highly dispersible liquid-phase support to improve
coupling efficiency is provided.The method for synthesizing an
oligonucleotide comprising: sequentially condensing and oxidizing a
nucleoside phosphoramidite compound in the presence of an acid /
azole complex compound using a starting
raw material, i.e., hydrophobic group-bonded
nucleoside represented by Formula (1):where R1: an alkylene group having 1 to 12 carbon atoms, R2: an alkylene group having 1 to 22 carbon atoms, R3 and R4 each independently represent an
alkyl group having 1 to 22 carbon atoms or the like, R5: a
single bond or an alkylene group having 1 to 22 carbon atoms, R6: each independently an
alkyl group having 6 to 30 carbon atoms, n represents an integer of 2 to 6, X represents a
hydrogen atom, hydroxyl group, or the like, Y: a
protecting group deprotectable under an acidic condition, and Z: an adenyl group, a guanyl group, or the like having a polar group optionally protected by a
protecting group,wherein a
condensation reaction is performed by preliminarily dissolving the hydrophobic group-bonded
nucleoside or hydrophobic group-bonded
oligonucleotide and the nucleoside
phosphoramidite compound in a non-polar
solvent, and contacting the resulting solution with the acid /
azole complex compound or a solution containing the complex compound.