Provided are: an oligonucleotide having, at 5-terminal thereof, a nucleotide or nucleoside residue represented by formula (I) {wherein X1 represents an oxygen atom or the like R1 represents formula (IIA) (wherein R5A represents a halogen atom or the like and R6A represents a hydrogen atom or the like), formula (IVA) (wherein Y3A represents a nitrogen atom or the like and Y4A represents CH or the like) or the like R2 represents a hydrogen atom, a hydroxy group, a halogen atom or a lower alkoxy group which may have a substituent and R3 represents a hydrogen atom or formula (VI) (wherein n2 represents 1, 2 or 3)}, wherein the nucleotide or nucleoside residue is bound to an adjacent nucleotide residue through an oxygen atom that is located at position-3 in the nucleotide or nucleoside residue and others.
