The purpose of the present invention is therefore to provide a method for binding a 3'-end nucleoside unit comprising any base to a hydroxyl group on a solid-phase support under completely the same condition as in DNA chain elongation reaction. The present invention relates to a 3'-end nucleoside unit comprising phosphoramidite that is a compound represented by the following formula: (N)--O--(R1)Si(R2)-(C.sub.6H.sub.4)--(CH.sub.2)n-O--P(OR3)N(R4)(R5) (I) wherein (N) represents any nucleoside or its derivative, each of R1, R2, R4 and R5 is an alkyl or aryl group, R3 is a phosphate-protecting group, and n is an integer of from 1 to 5; a solid-phase support having said 3'-end nucleoside unit; and a method for the synthesis of a nucleic acid oligomer with the use of said solid-phase support.