A method of easily releasing a useful substance bonded to oligonucleotide without impairing a target nucleic acid; and a novel base therefore. A nucleoside of nucleotide (oligonucleotide containing thereof) represented by the formula (I) (wherein each of X and Y independently represents --O--, --NH--, --N(alkyl)- or --S--; R represents a functional unit, a reporter unit or a biofunctional molecule; each of R.sup.1 and R.sup.2 independently represents a hydrogen atom, a phosphate bond group, a phosphoramidite group or a nucleotide; and n is a numeral of 1 to 10). There is further provided a method of releasing the R group moiety at base portion by the use of the oligonucleotide comprising the nucleotide.