An oligonucleotide comprising a protected base, which is represented by the formula: wherein q is any integer of not less than 0; Base 2 in the number of q+1 are each independently a nucleic acid base protected by a group represented by the formula (k): wherein * indicates the bonding position to a nucleic acid base; R 27 is a C 5-30 straight chain or branched chain alkyl group or a C 5-30 straight chain or branched chain alkenyl group, a group represented by the formula: wherein * indicates the bonding position to a nucleic acid base; Q 1 is -0-, -S- or -NR 30 - wherein R 30 is a hydrogen atom or a C 1-22 alkyl group; R c and R d are each independently a hydrogen atom or a C 1-22 alkyl group; and R 28 is a C 5-30 straight chain or branched chain alkyl group or a C 5-30 straight chain or branched chain alkenyl group, a group represented by the formula (m): wherein * indicates the bonding position to a nucleic acid base; 1 is an integer of 1 to 5; Q 2 in the number of 1 are each independently a single bond, or -0-, -S-, -OC(=O)-, -C(=O)O-, -O-CH 2 -, -NH-, -NHC(=O)-, -C(=O)NH-, -NH-CH 2 - or -CH 2 -; R 29 in the number of 1 are each independently a C 5-30 straight chain or branched chain alkyl group or a C 5-30 straight chain or branched chain alkenyl group; ring C is a benzene ring or a cyclohexane ring, each optionally having, in addition to Q 2 R 29 in the number of 1 and *C=O, a substituent selected from the group consisting of a halogen atom, a C 1-6 alkyl group optionally substituted by one or more halogen atoms, and a C 1-6 alkoxy group optionally substituted by one or more halogen atoms, or a group represented by the formula (s): wherein * indicates the position at which an imino bond is formed with an amino group of a nucleic acid base; and R 35 and R 36 are each independently a C 5-30 straight chain or branched chain alkyl group or a C 5-30 straight chain or branched chain alkenyl group; P 1 is a hydrogen atom, or a temporary protecting group removable under acidic conditions; X is a hydrogen atom, an optionally protected hydroxyl group, a halogen atom or an organic group crosslinked with the 4-position carbon atom; X' in the number of q are each independently a hydrogen atom, an optionally protected hydroxyl group, a halogen atom or an organic group crosslinked with the 4-position carbon atom; P 2 in the number of q+1 are each independently a protecting group removable under basic conditions; R 34 in the number of q are each independently an oxygen atom or a sulfur atom; and R e and R f are each independently a C 1-6 alkyl group, or a 5- or 6-membered saturated cyclic amino group formed together with the adjacent nitrogen atom. wherein q is any integer of not less than 0; Base 2 in the number of q+1 are each independently a nucleic acid base protected by a group represented by the formula (k): wherein * indicates the bonding position to a nucleic acid base; R 27 is a C 5-30 straight chain or branched chain alkyl group or a C 5-30 straight chain or branched chain alkenyl group, a group represented by the formula: wherein * indicates the bonding position to a nucleic acid base; Q 1 is -0-, -S- or -NR 30 - wherein R 30 is a hydrogen atom or a C 1-22 alkyl group; R c and R d are each independently a hydrogen atom or a C 1-22 alkyl group; and R 28 is a C 5-30 straight chain or branched chain alkyl group or a C 5-30 straight chain or branched chain alkenyl group, a group represented by the formula (m): wherein * indicates the bonding position to a nucleic acid base; 1 is an integer of 1 to 5; Q 2 in the number of 1 are each independently a single bond, or -0-, -S-, -OC(=O)-, -C(=O)O-, -O-CH 2 -, -NH-, -NHC(=O)-, -C(=O)NH-, -NH-CH 2 - or -CH 2 -; R 29 in the number of 1 are each independently a C 5-30 straight chain or branched chain alkyl group or a C 5-30 straight chain or branched chain alkenyl group; ring C is a benzene ring or a cyclohexane ring, each optionally having, in addition to Q 2 R 29 in the number of 1 and *C=O, a substituent selected from the group consisting of a halogen atom, a C 1-6 alkyl group optionally substituted by one or more halogen atoms, and a C 1-6 alkoxy group optionally substituted by one or more halogen atoms, or a group represented by the formula (s): wherein * indicates the position at which an imino bond is formed with an amino group of a nucleic acid base; and R 35 and R 36 are each independently a C 5-30 straight chain or branched chain alkyl group or a C 5-30 straight chain or branched chain alkenyl group; P 1 is a hydrogen atom, or a temporary protecting group removable under acidic conditions; X is a hydrogen atom, an optionally protected hydroxyl group, a halogen atom or an organic group crosslinked with the 4-position carbon atom; X' in the number of q are each independently a hydrogen atom, an optionally protected hydroxyl group, a halogen atom or an organic group crosslinked with the 4-position carbon atom; P 2 in the number of q+1 are each independently a protecting group removable under basic conditions; R 34 in the number of q are each independently an oxygen atom or a sulfur atom; and R e and R f are each independently a C 1-6 alkyl group, or a 5- or 6-membered saturated cyclic amino group formed together with the adjacent nitrogen atom.