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.