Methods of synthesizing nucleic acid oligomers on a solid-phase support having a 3′-end nucleoside unit introduced thereon-as represented by formula II: wherein of formula II represents a 2′-deoxyribonucleoside or its N-protected derivative, the substituent —O—(R1)Si(R2)—(C6H3R6)—(CH2)n—O—P(OR3)XO)—(CH2)n is attached at the 3′ position of the sugar moiety of the nucleoside substituent; each of R1 and R2 is an alkyl or optionally substituted aryl group, wherein the optionally substituted aryl group has a substituent selected from the group consisting of C1-4 alkyl, nitro, cyano, halo and methoxyl; R3 is a protecting group; X is S or O; R7 is H or 4,4′-dimethoxytrityl; each n is an integer of from 1 to 5; and the solid-phase support has hydroxyl groups on its surface.