PURPOSE: To obtain new 2-[2-(monomethoxytrityloxy)ethylthio]ethyl group useful as a protecting group of phosphoric acid residue used in synthesizing an oligomer having the phosphoric acid residue at the 3'- and/or 5'-terminals.

CONSTITUTION: 2-[2-(Monomethoxytrityloxy)ethylthio]ethyl group expressed by formula I (MMTr is monomethoxytrityl). The protecting group expressed by formula I is prepared by reacting 2,2'-thiodiethanol with a monomethoxytrityl halide (e.g. monomethoxytrityl chloride) in a reaction solvent at 0-100°C. The protecting group can be used for synthesizing DNA type or RNA type oligonucleotides having phosphoric acid residue at the 3'- and/or 5'=terminals. The protecting group is initially introduced into the 3'- and/or 5'-terminals of nucleotides and then reacted according to, e.g. a phosphoric acid triester method to afford the objective oligonucleotides, e.g. uridine derivatives expressed by formula II.