1. A production method of an oligonucleotide comprising a step of subjecting a 5'-hydroxyl group or 3'-hydroxyl group of a nucleoside or oligonucleotide having a pseudo solid phase-protecting group in at least one location selected from the group consisting of 2'-position, 3'-position, 5'-position and a nucleobase moiety and having a 5'-hydroxyl group or a 3'-hydroxyl group, to H-phosphonation to convert the 5'-hydroxyl group or the 3'-hydroxyl group into an H-phosphonated form.
2. The production method according to Claim 1, which comprises at least one elongation reaction cycle comprising a step of subjecting a nucleoside or oligonucleotide having a pseudo solid phase-protecting group in at least one location selected from the group consisting of 2'-position, 3'-position, 5'-position and a nucleobase moiety and having a 5'-hydroxyl group or a 3'-hydroxyl group, to H-phosphonation to convert the 5'-hydroxyl group or the 3'-hydroxyl group into an H-phosphonated form.
3. The production method according to Claim 2, wherein the elongation reaction cycle comprises:
a first step including deprotecting a first nucleoside or first oligonucleotide having a pseudo solid phase-protecting group in at least one location selected from the group consisting of 2'-position, 3'-position and a nucleobase moiety, having a 3'-hydroxyl group protected with a basic protecting group or a pseudo solid phase-protecting group, and having a 5'-hydroxyl group protected with a temporary protecting group, to remove the temporary protecting group to form a 5'-hydroxyl group,
a second step including converting the resultant 5'-hydroxyl group into an H-phosphonated form using an H-phosphonate reagent, and
a third step of forming an oligomer of the first nucleoside or first oligonucleotide with a second nucleoside or second oligonucleotide having a 3'-hydroxyl group and having a 5'-hydroxyl group protected with a temporary protecting group, by forming a phosphite diester bond from the 5'-hydroxyl group, now converted to the H-phosphonated form, of the first nucleoside or first oligonucleotide and the 3'-hydroxyl group of the second nucleoside or second oligonucleotide.
4. The production method according to Claim 3, further comprising a fourth step

including converting the phosphite diester bond of the oligomer into a phosphodiester bond, a thiophosphodiester bond, a boranophosphodiester bond, an aminophosphodiester bond, a phosphodiester bond protected with a basic protecting group, or a thiophosphodiester bond protected with a basic protecting group.
5. The production method according to Claim 3, further comprising a fourth step including converting the phosphite diester bond of the oligomer into a phosphodiester bond, a thiophosphodiester bond, a boranophosphodiester bond or an aminophosphodiester bond.
6. The production method according to Claim 4 or 5, further comprising a fifth step of adding a polar solvent to a reaction mixture obtained from at least one step selected from the group consisting of the first step to the fourth step to form a precipitate, and collecting the precipitate by solid liquid separation.
7. The production method according to Claim 6, wherein the polar solvent is an alcohol solvent with 1 to 6 carbon atoms or a nitrile solvent with 1 to 6 carbon atoms.
8. The production method according to any one of Claims 3 to 7, further comprising a sixth step of removing all of the basic protecting group, the temporary protecting group and the pseudo solid phase-protecting group.
9. The production method according to any one of Claims 3 to 8, wherein the first nucleoside or the first oligonucleotide has a hydroxyl group protected with a pseudo solid phase-protecting group at 3'-position.
10. The production method according to any one of Claims 3 to 9, wherein the third step uses the second nucleoside.
11. The production method according to Claim 2, wherein the elongation reaction cycle comprises:
a seventh step including deprotecting a third nucleoside or third oligonucleotide having a pseudo solid phase-protecting group in at least one location selected from the group consisting of 2'-position, 5'-position and a nucleobase moiety, having a 5'-hydroxyl group protected with a basic protecting group or a pseudo solid phase-protecting group, and having a 3'-hydroxyl group protected with a temporary

protecting group, to remove the temporary protecting group to form a 3'-hydroxyl group,
an eighth step including converting the resultant 3'-hydroxyl group into an H-phosphonated form using an H-phosphonate reagent, and
a ninth step of forming an oligomer of the third nucleoside or third oligonucleotide with a fourth nucleoside or fourth oligonucleotide having a 5'-hydroxyl group and having a 3'-hydroxyl group protected with a temporary protecting group, by forming a phosphite diester bond from the 3'-hydroxyl group, now converted to the H-phosphonated form, of the third nucleoside or third oligonucleotide and the 5'-hydroxyl group of the fourth nucleoside or fourth oligonucleotide.
12. The production method according to Claim 11, further comprising a tenth step including converting the phosphite diester bond of the oligomer into a phosphodiester bond, a thiophosphodiester bond, a boranophosphodiester bond, an aminophosphodiester bond, a phosphodiester bond protected with a basic protecting group, or a thiophosphodiester bond protected with a basic protecting group.
13. The production method according to Claim 11, further comprising a tenth step including converting the phosphite diester bond of the oligomer into a phosphodiester bond, a thiophosphodiester bond, a boranophosphodiester bond or an aminophosphodiester bond.
14. The production method according to Claim 12 or 13, further comprising an eleventh step of adding a polar solvent to a reaction mixture obtained from any of the seventh step to the tenth step to form a precipitate, and collecting the precipitate by solid liquid separation.
15. The production method according to Claim 14, wherein the polar solvent is an alcohol solvent with 1 to 6 carbon atoms or a nitrile solvent with 1 to 6 carbon atoms.
16. The production method according to any one of Claims 11 to 15, further comprising a twelfth step of removing all of the basic protecting group, the temporary protecting group and the pseudo solid phase-protecting group.
17. The production method according to any one of Claims 11 to 16, wherein the third nucleoside or the third oligonucleotide has a hydroxyl group protected with a

pseudo solid phase-protecting group at 5'-position.
18. The production method according to any one of Claims 11 to 17, wherein the ninth step uses the fourth nucleoside.
19. The production method according to any one of Claims 1 to 18, wherein the pseudo solid phase-protecting group is represented by the following formula (I):
wherein * indicates a bond to a group protected by the pseudo solid phase-protecting group,
R1 is a CI-40 alkyl group, a C2-40 alkenyl group or a C2-40 alkynyl group, s is an integer of 1 to 5,
L1 is a single bond, a CI-6 alkylene group, a C2-6 alkenylene group or a C2-6 alkynylene group,
L2 is a single bond, -COO-, -CON(R2)- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -OCO-, -N(R2)CO- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -C(O)- or -0-,
L3 is a single bond, a CI-6 alkylene group, a C2-6 alkenyl ene group or a C2-6 alkynylene group,
L4 is a single bond, -COO-, -CON(R2)- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -OCO-, -N(R2)CO- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -C(O)- or -O-,
L5 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group,
L6 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group,
mis 0 or 1, and
when the pseudo solid phase-protecting group is present in the nucleobase moiety and when m in the pseudo solid phase-protecting group is 0, the number of single bonds represented by L1, L2, L3 and L4 in the pseudo solid phase-protecting group is 0 to 3.

20. The production method according to any one of Claims 1 to 18, wherein the
pseudo solid phase-protecting group is represented by the following formula (II):
wherein * indicates a bond to a group protected by the pseudo solid phase-protecting group,
R1 is a CI-40 alkyl group, a C2-40 alkenyl group or a C2-40 alkynyl group, s is an integer of 1 to 5,
L1 is a single bond, a CI-6 alkylene group, a C2-6 alkenylene group or a C2-6 alkynylene group,
9 9 9
L is a single bond, -COO-, -CON(R )- (wherein R is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group),
9 9
-OCO-, -N(R )CO- (wherein R is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group) or -0-,
L is a single bond, a CI-6 alkylene group, a C2-6 alkenyl ene group or a C2-6 alkynylene group,
A 9 9
L is a single bond, -COO-, -CON(R )- (wherein R is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group),
9 9
-OCO-, -N(R )CO- (wherein R is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group) or -0-, and
when the pseudo solid phase-protecting group is present in the nucleobase moiety, the number of single bonds represented by L1, L2, L3 and L4 in the pseudo solid phase-protecting group is 0 to 3.
21. The production method according to any one of Claims 3 to 20, wherein the temporary protecting group is a tert-butyldimethylsilyl group, a 4,4'-dimethoxytrityl group or a levulinyl group.
22. The production method according to any one of Claims 1 to 21, wherein the H-phosphonation step uses at least one H-phosphonate reagent selected from the group consisting of phosphorous acid, diphenyl phosphite, phenyl-H-phosphonate triethylammonium salt, p-toluyl-H-phosphonate triethylammonium salt, 2-chloro-4H-l,3,2-benzodioxaphosphorin-4-one and phosphorus trichloride.

23. The production method according to any one of Claims 1 to 22, wherein the nucleobases present in the first to the fourth nucleosides and the first to the fourth oligonucleotides are each independently at least one selected from the group consisting of 6-aminopurin-9-yl group (adenine residue), 2-amino-6-hydroxypurin-9-yl group (guanine residue), 2-oxo-4-amino-l,2-dihydropyrimidin-l-yl group (cytosine residue), 2-oxo-4-amino-5-methyl- 1,2-dihydropyrimidin-l-yl group (5-methylcytosine residue), 2-oxo-4-hydroxy-l,2-dihydropyrimidin-l-yl group (uracil residue) and 2-oxo-4-hydroxy-5-methyl-1,2-dihydropyrimidin-l-yl group (thymine residue).
24. A compound represented by the following formula (XI), or a salt thereof:
wherein n is an integer of 1 or greater,
Base2 independently at each occurrence is a nucleobase, a nucleobase protected with a basic protecting group, or a nucleobase protected with a pseudo solid phase-protecting group,
Xz independently at each occurrence is a hydrogen atom, a halogen atom, a hydroxyl group, a hydroxyl group protected with a basic protecting group, an organic group bridged to the 4-carbon atom, or a hydroxyl group protected with a pseudo solid phase-protecting group,
Y independently at each occurrence is a hydrogen atom, a hydroxyl group, a hydroxyl group protected with a basic protecting group, a thiol group, a thiol group protected with a basic protecting group, a borohydride group, a mono-Cl-6 alkylamino group or a di-Cl-6 alkylamino group,
Z is a hydrogen atom, a basic protecting group, a temporary protecting group or a pseudo solid phase-protecting group, and
the compound or salt thereof satisfies at least one of that at least one Base is a nucleobase protected with a pseudo solid phase-protecting group, that at least one X is a hydroxyl group protected with a pseudo solid phase-protecting group, and that Z is a

pseudo solid phase-protecting group.
25. The compound or salt thereof according to Claim 24, wherein the pseudo solid
phase-protecting group is represented by the following formula (II):
wherein * indicates a bond to a group protected by the pseudo solid phase-protecting group,
R1 is a CI-40 alkyl group, a C2-40 alkenyl group or a C2-40 alkynyl group, s is an integer of 1 to 5,
L1 is a single bond, a CI-6 alkylene group, a C2-6 alkenylene group or a C2-6 alkynylene group,
L2 is a single bond, -COO-, -CON(R2)- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -OCO-, -N(R2)CO- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group) or -O-,
L3 is a single bond, a CI-6 alkylene group, a C2-6 alkenyl ene group or a C2-6 alkynylene group,
L4 is a single bond, -COO-, -CON(R2)- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -OCO-, -N(R2)CO- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group) or -O-, and
when the pseudo solid phase-protecting group is present in the nucleobase moiety, the number of single bonds represented by L1, L2, L3 and L4 in the pseudo solid phase-protecting group is 0 to 3.
26. The compound or salt thereof according to Claim 24 or 25, which satisfies at least one of that at least one Base in the formula (XI) is a nucleobase protected with a pseudo solid phase-protecting group, and that Z is a pseudo solid phase-protecting group.
27. The compound or salt thereof according to any one of Claims 24 to 26, wherein Z in the formula (XI) is a pseudo solid phase-protecting group.

28. The compound or salt thereof according to any one of Claims 24 to 27, wherein n in the formula (XI) is 1 to 30.
29. A compound represented by the following formula (XII), or a salt thereof:
wherein
n is an integer of 1 or greater,
Base independently at each occurrence is a nucleobase, a nucleobase protected with a basic protecting group, or a nucleobase protected with a pseudo solid phase-protecting group,
Xz independently at each occurrence is a hydrogen atom, a halogen atom, a hydroxyl group, a hydroxyl group protected with a basic protecting group, an organic group bridged to the 4-carbon atom, or a hydroxyl group protected with a pseudo solid phase-protecting group,
Y independently at each occurrence is a hydrogen atom, a hydroxyl group, a hydroxyl group protected with a basic protecting group, a thiol group, a thiol group protected with a basic protecting group, a borohydride group, a mono-Cl-6 alkylamino group or a di-Cl-6 alkylamino group,
Z is a hydrogen atom, a basic protecting group, a temporary protecting group or a pseudo solid phase-protecting group, and
the compound or salt thereof satisfies at least one of that at least one Base is a nucleobase protected with a pseudo solid phase-protecting group, that at least one Xz is a hydroxyl group protected with a pseudo solid phase-protecting group, and that Z is a pseudo solid phase-protecting group.
30. The compound or salt thereof according to Claim 29, wherein the pseudo solid
phase-protecting group is represented by the following formula (II):

wherein * indicates a bond to a group protected by the pseudo solid phase-protecting group,
R1 is a CI-40 alkyl group, a C2-40 alkenyl group or a C2-40 alkynyl group, s is an integer of 1 to 5,
L1 is a single bond, a CI-6 alkylene group, a C2-6 alkenylene group or a C2-6 alkynylene group,
L2 is a single bond, -COO-, -CON(R2)- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -OCO-, -N(R2)CO- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group) or -O-,
L is a single bond, a CI-6 alkylene group, a C2-6 alkenyl ene group or a C2-6 alkynylene group,
L4 is a single bond, -COO-, -CON(R2)- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -OCO-, -N(R2)CO- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group) or -0-, and
when the pseudo solid phase-protecting group is present in the nucleobase moiety, the number of single bonds represented by L1, L2, L3 and L4 in the pseudo solid phase-protecting group is 0 to 3.
31. The compound or salt thereof according to Claim 29 or 30, which satisfies at least one of that at least one Base2 in the formula (XII) is a nucleobase protected with a pseudo solid phase-protecting group, and that Z is a pseudo solid phase-protecting group.
32. The compound or salt thereof according to any one of Claims 29 to 31, wherein Z in the formula (XII) is a pseudo solid phase-protecting group.
33. The compound or salt thereof according to any one of Claims 29 to 32, wherein n in the formula (XII) is 1 to 30.
34. A compound represented by the following formula (XIII), or a salt thereof:

wherein n is an integer of 1 or greater,
Base2 independently at each occurrence is a nucleobase, a nucleobase protected with a basic protecting group, or a nucleobase protected with a pseudo solid phase-protecting group,
Xz independently at each occurrence is a hydrogen atom, a halogen atom, a hydroxyl group, a hydroxyl group protected with a basic protecting group, an organic group bridged to the 4-carbon atom, or a hydroxyl group protected with a pseudo solid phase-protecting group,
W is a hydrogen atom or a temporary protecting group,
Y independently at each occurrence is a hydrogen atom, a hydroxyl group, a hydroxyl group protected with a basic protecting group, a thiol group, a thiol group protected with a basic protecting group, a borohydride group, a mono-Cl-6 alkylamino group or a di-Cl-6 alkylamino group,
Z is a hydrogen atom, a basic protecting group, a temporary protecting group or a pseudo solid phase-protecting group,
the compound or salt thereof satisfies at least one of that at least one Base is a nucleobase protected with a pseudo solid phase-protecting group, that at least one xz IS a hydroxyl group protected with a pseudo solid phase-protecting group, and that Z is a pseudo solid phase-protecting group, and
at least one of the pseudo solid phase-protecting groups is represented by the formula (I):
wherein * indicates a bond to a group protected by the pseudo solid phase-protecting group,
R1 is a CI-40 alkyl group, a C2-40 alkenyl group or a C2-40 alkynyl group, s is

an integer of 1 to 5,
L1 is a single bond, a CI-6 alkylene group, a C2-6 alkenylene group or a C2-6 alkynylene group,
L2 is a single bond, -COO-, -CON(R2)- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group),
9 9
-OCO-, -N(R )CO- (wherein R is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -C(O)- or -0-,
L3 is a single bond, a CI-6 alkylene group, a C2-6 alkenylene group or a C2-6 alkynylene group,
L4 is a single bond, -COO-, -CON(R2)- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -OCO-, -N(R2)CO- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -C(O)- or -0-,
L5 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group,
L6 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group, and
m is 0 or 1 with the proviso that when m is 0, L4 is not a single bond.
35. A compound represented by the following formula (XIV), or a salt thereof:
wherein
n is an integer of 1 or greater,
Base2 independently at each occurrence is a nucleobase, a nucleobase protected with a basic protecting group, or a nucleobase protected with a pseudo solid phase-protecting group,
Xz independently at each occurrence is a hydrogen atom, a halogen atom, a hydroxyl group, a hydroxyl group protected with a basic protecting group, an organic group bridged to the 4-carbon atom, or a hydroxyl group protected with a pseudo solid

phase-protecting group,
W is a hydrogen atom or a temporary protecting group,
Y independently at each occurrence is a hydrogen atom, a hydroxyl group, a hydroxyl group protected with a basic protecting group, a thiol group, a thiol group > protected with a basic protecting group, a borohydride group, a mono-Cl-6 alkylamino group or a di-Cl-6 alkylamino group,
Z is a hydrogen atom, a basic protecting group, a temporary protecting group or a pseudo solid phase-protecting group,
the compound or salt thereof satisfies at least one of that at least one Base is a ) nucleobase protected with a pseudo solid phase-protecting group, that at least one X is a hydroxyl group protected with a pseudo solid phase-protecting group, and that Z is a pseudo solid phase-protecting group, and
at least one of the pseudo solid phase-protecting groups is represented by the formula (I):
wherein * indicates a bond to a group protected by the pseudo solid phase-protecting group,
R1 is a CI-40 alkyl group, a C2-40 alkenyl group or a C2-40 alkynyl group, s is an integer of 1 to 5,
L1 is a single bond, a CI-6 alkylene group, a C2-6 alkenylene group or a C2-6 alkynylene group,
L2 is a single bond, -COO-, -CON(R2)- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group),
9 9
-OCO-, -N(R )CO- (wherein R is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -C(O)- or -O-,
L3 is a single bond, a CI-6 alkylene group, a C2-6 alkenylene group or a C2-6 alkynylene group,
A 9 9
L is a single bond, -COO-, -CON(R )- (wherein R is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -OCO-, -N(R2)CO- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -C(O)- or -0-,
L5 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group,

L6 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group, and
m is 0 or 1 with the proviso that when m is 0, L4 is not a single bond.
36. A pseudo solid phase-protecting group represented by the following formula
(I):
wherein * indicates a bond to at least one selected from the group consisting of 2'-, 3'- and 5'-hydroxyl groups and a nucleobase moiety of a nucleoside or oligonucleotide,
R1 is a CI-40 alkyl group, a C2-40 alkenyl group or a C2-40 alkynyl group, s is an integer of 1 to 5,
L1 is a single bond, a CI-6 alkylene group, a C2-6 alkenylene group or a C2-6 alkynylene group,
L2 is a single bond, -COO-, -CON(R2)- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -OCO-, -N(R )CO- (wherein R is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -C(O)- or -O-,
L is a single bond, a CI-6 alkylene group, a C2-6 alkenyl ene group or a C2-6 alkynylene group,
L4 is a single bond, -COO-, -CON(R2)- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -OCO-, -N(R2)CO- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -C(O)- or -O-,
L5 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group,
L6 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group, and
m is 0 or 1 with the proviso that when m is 0, L4 is not a single bond.
37. A method for producing a nucleoside or oligonucleotide, comprising reacting a
nucleoside or oligonucleotide having:
a hydroxyl group or a protected hydroxyl group independently at 3'-position

and 5'-position, and
a hydroxyl group in at least one location selected from the group consisting of 2'-position, 3'-position, 5'-position and a nucleobase moiety, or a hydroxyl or amino group in at least one location of 2'-position and a nucleobase moiety, with
a carboxyl compound represented by the following formula (X-l):
wherein R is a CI-40 alkyl group, a C2-40 alkenyl group or a C2-40 alkynyl group, s is an integer of 1 to 5,
L1 is a single bond, a CI-6 alkylene group, a C2-6 alkenylene group or a C2-6 alkynylene group,
L2 is a single bond, -COO-, -CON(R2)- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -OCO-, -N(R2)CO- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -C(O)- or -0-,
L is a single bond, a CI-6 alkylene group, a C2-6 alkenyl ene group or a C2-6 alkynylene group,
L4 is -COO-, -CON(R2)- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -OCO-, -N(R2)CO- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -C(O)- or -0-, or with
an acid halide represented by the following formula (X-2):
wherein X is a halogen atom, and R1, L1, L2, L3 and L4 are the same as defined in the formula (X-l), or with
an alkyl halide compound represented by the following formula (X-3):

wherein X is a halogen atom, R1, L1, L2 and L3 are the same as defined in the formula (X-l),
L4 is a single bond, -COO-, -CON(R2)- (wherein R2 is a hydrogen atom, a CI-6
> alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group),
-OCO-, -N(R )CO- (wherein R is a hydrogen atom, a CI-6 alkyl group, a CI-6
haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -C(O)- or -0-,
L5 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6
alkenyl group or a C2-6 haloalkenyl group, and
) L6 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6
alkenyl group or a C2-6 haloalkenyl group; thereby producing a nucleoside or oligonucleotide having, in at least one location selected from the group consisting of 2'-position, 3'-position, 5'-position and a nucleobase moiety, a pseudo solid phase-protecting group represented by the following formula (I):
wherein * indicates a bond to at least one selected from the group consisting of the 2'-position, the 3'-position, the 5'-position and the nucleobase moiety,
R1 is a CI-40 alkyl group, a C2-40 alkenyl group or a C2-40 alkynyl group, s is an integer of 1 to 5,
L1 is a single bond, a CI-6 alkylene group, a C2-6 alkenylene group or a C2-6 alkynylene group,
L2 is a single bond, -COO-, -CON(R2)- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -OCO-, -N(R2)CO- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -C(O)- or -0-,
L3 is a single bond, a CI-6 alkylene group, a C2-6 alkenylene group or a C2-6 alkynylene group,
L4 is a single bond, -COO-, -CON(R2)- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group),

-0C0-, -N(R2)CO- (wherein R2 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group), -C(O)- or -0-,
L5 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group,
L6 is a hydrogen atom, a CI-6 alkyl group, a CI-6 haloalkyl group, a C2-6 alkenyl group or a C2-6 haloalkenyl group, and
m is 0 or 1 with the proviso that when m is 0, L4 is not a single bond.