In this method, an oligonucleotide is prepared by using, as a synthesis unit, a novel nucleoside monomer compound represented by formula (I) [wherein X, R1, Y, Base, Z, Ar, R2, R3 and n are each as defined in Claim 1]. The novel nucleoside monomer compound is a nucleoside, the base moiety of which is substituted with an aromatic-hydrocarbon-ring-carbonyl or -thiocarbonyl group having at least one hydrophobic group. The method cars dispense with column-chromatographic purification in every reaction, and enables base elongation not only in the 3′-direction but also in the 5′-direction, thus attaining efficient liquid-phase mass synthesis of an oligonucleotide.