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WHAT IS CLAIMED IS:
1. A thionucleoside derivative represented by General Formula [1]:

in the formula,
R represents a hydroxyl group which may be protected, a C1-20 alkoxy group which may be substituted, a C3-8 cycloalkoxy group which may be substituted, an aryloxy group which may be substituted, a heterocyclic oxy group which may be substituted, or an amino group which may be substituted;
R2 represents a Ci.2o alkoxy group which may be substituted, a C3-8 cycloalkoxy group which may be substituted, an aryloxy group which may be substituted, a heterocyclic oxy group which may be substituted, or an amino group which may be substituted; or
R and R , together with the phosphorus atom to which they are bonded, may form a 5- to 10-membered nitrogen'phosphorus-containing heterocyclic ring which may be substituted, a 5- to 10-membered oxygen-phosphorus-containing heterocyclic ring which may be substituted, or a 5- to 10-membered nitrogen■oxygemphosphorus-containing heterocyclic ring; and R3 represents a hydrogen atom; or
R and R , together with the phosphorus atom to which R is bonded, an oxygen atom, methylene, two carbon atoms constituting the tetrahydrothiophene ring and the oxygen atom to which R3 is bonded, may form a 6 to 10-membered oxygen-phosphorus-containing

heterocyclic ring which may be substituted; or a salt thereof.
2. The thionucleoside derivative according to claim 1, wherein R is a hydroxyl group which may be protected, a Ci-e alkoxy group which may be substituted with one or more substituents selected from Substituent group A, an aryloxy group which may be substituted with one or more substituents selected from Substituent group A, a heterocyclic oxy group which may be substituted with one or more substituents selected from Substituent group A, or an amino group which may be substituted with one or more substituents selected from Substituent group A; or the salt thereof.
Substituent group A:
A halogen atom; a hydroxyl group which may be protected; a cyano group; a nitro group; a carbamoyl group; an oxo group; a Ci_6 alkyl group which may be substituted with one or more substituents selected from Substituent group B; a C].6 alkyldisulfanyl group which may be substituted with one or more substituents selected from Substituent group B; a C2-6 alkenyl group which may be substituted with one or more substituents selected from Substituent group B; a C2-6 alkynyl group which may be substituted with one or more substituents selected from Substituent group B; a C3-8 cycloalkyl group which may be substituted with one or more substituents selected from Substituent group B; a C3.8 cycloalkyldisulfanyl group which may be substituted with one or more substituents selected from Substituent group B; a C^e alkoxy group which may be substituted with one or more substituents selected from Substituent group B; a C]_6 alkoxycarbonyloxy group which may be substituted with one or more substituents selected from Substituent group B; a C3-3 cycloalkoxycarbonyl group which may be substituted with one or more substituents selected from Substituent group B; an aryl group which may be substituted with one or more

substituents selected from Substituent group B; an aryldisulfanyl group which may be substituted with one or more substituents selected from Substituent group B; a heterocyclic group which may be substituted with one or more substituents selected from Substituent group B; a heterocyclic oxy group which may be substituted with one or more substituents selected from Substituent group B; an acyloxy group which may be substituted with one or more substituents selected from Substituent group B; an acylthio group which may be substituted with one or more substituents selected from Substituent group B; an aminocarbonyioxy group which may be substituted with one or more substituents selected from Substituent group B; and an aminocarbonylthio group which may be substituted with one or more substituents selected from Substituent group B.
Substituent group B:
A halogen atom; a hydroxyl group; a cyano group; a nitro group; a carboxyl group; a carbamoyl group; a hydroxymethyl group; a Cj-6 alkyl group; a C2-6 alkenyl group; a C2-6 alkynyl group; a C3.8 cycloalkyl group; a C\-e alkoxy group; a d-6 alkoxycarbonyl group; a C3.8 cycloalkoxycarbonyl group; an ar-Ci-e alkoxycarbonyl group; an aryl group; an aryloxy group; a heterocyclic oxy group which may be substituted with one or more substituents selected from a hydroxyl group and a hydroxymethyl group; an ar-Q.6 alkoxy group; and an acyloxy group.
3. The thionucleoside derivative according to claim 1 or 2, wherein R2 is a C1-6 alkoxy group which may be substituted with one or more substituents selected from Substituent group A, an aryloxy group which may be substituted with one or more substituents selected from Substituent group A, or an amino group which may be substituted with one or more substituents selected from Substituent group A; or the salt thereof.

4. The thionucleoside derivative according to any one of claims 1 to 3, wherein R1 is a hydroxy! group which may be protected, a C].6 alkoxy group which may be substituted with one or more substituents selected from Substituent group A, an aryloxy group which may be substituted with one or more substituents selected from Substituent group A, a heterocyclic oxy group which may be substituted with one or more substituents selected from Substituent group A, or an amino group which may be substituted with one or more substituents selected from Substituent group A;and R2 is a Cj-6 alkoxy group which may be substituted with one or more substituents selected from Substituent group A, an aryloxy group which may be substituted with one or more substituents selected from Substituent group A, or an amino group which may be substituted with one or more substituents selected from Substituent group A; or the salt thereof.
5. The thionucleoside derivative according to any one of claims 1 to 4, wherein R is an aryloxy group which may be substituted with one or more substituents selected from Substituent group A, and R2 is an amino group which may be substituted with one or more substituents selected from Substituent group A, or
R is a Ci_6 alkoxy group which may be substituted with one or more substituents selected from Substituent group A, and R is a C\.e alkoxy group which may be substituted with one or more substituents selected from Substituent group A; or the salt thereof.
6. The thionucleoside derivative according to claim 1, wherein the thionucleoside derivative
is a compound selected from:
(2S)-benzyl

2-(((RS)-t((2R,3S,4S,5R)-5-(4-amino-2-oxopyrimidin-l(2H)-yl)-4-fIuoro-3-hydroxytetrahydr othiophen-2-yI)methoxy)(phenoxy)phosphoryI)amino)propanoate.
(2S)-methyl 2-(((RS)-(((2R,3S,4S,5R)-5-(4-amino-2-oxopyrimidin-l(2H)-yl)-4-fluoro-3-hydroxytetrahydr othiophen-2-yl)methoxy)t4-chlorophenoxy)phosphoryl)amino)propanoate,
(2S)-cyclobutyl 2-(C(RS)-(((2R,3S,4S,5R)-5-(4-amino-2-oxopyrimidin-l(2H)-yl)-4-fluoro-3-hydroxytelrahydr othiophen-2-yl)methoxy)(phenoxy)phosphoryl)amino)propanoate,
(2S)-methyl 2-(((RS)-(((2R,3S,4S,5R)-5-(4-amino-2-oxopyrimidin-l(2H)-yI)-4-fluoro-3-hydroxytetraliydr othiophen-2-yl)methoxy)(4-bromophenoxy)phosphoryl)amino)propanoate,
(2S)-methyl 2-(((RS)-(((2R,3S,4S,5R)-5-(4-amino-2-oxopyrimidin-l(2H)-yl)-4-fluoro-3-hydroxytetrahydr othiophen-2-yl)methoxy)(4-iodophenoxy)phosphoryI)amino)propanoate,
((2R,3S,4S,5R)-5-(4-amino-2-oxopyrimidin-l(2H)-yl)-4-fluoro-3-hydroxytetrahydrot hiophen-2-yl)methyl bis(S-pivaloyl-2-mercaptoethan-l -yl) phosphate,
((2Rs3S,4S,5R)-5-(4-amino-2-oxopyrimidin-l(2H)-yl)-4-fluoro-3-hydroxytetrahydrot hiophen-2-yl)methyl bis(S-isobutyryl-2-mercaptoethan-1 -yl) phosphate,
((2R,3S,4S,5R)-5-(4-amino-2-oxopyrimidin-l(2H)-yl)-4-fluoro-3-hydroxytetrahydrot hiophen-2-yl)methyl bis(S-propionyl-2-mercaptoethan-1 -yl) phosphate,
(((RS)-C((2R,3S,4S,5R)-5-(4-amino-2-oxopyrimidin-l(2H)-yl)-4-iluoro-3-hydroxytetr ahydrothiophen-2-yl)methoxy)(2-(pivaloylthio)ethoxy)phosphoryl)oxy)methyl pivalate,
S-(2-(((RS)-(((2R,3S,4S,5R)-5-(4-amino~2-oxopyrimidin-l(2n)-yl)-4-fluoro-3-hydro xytetrahydrothiophen-2-yl)methoxy)(((isopropoxycarbonyl)oxy)methoxy)phosphoryl)oxy)eth

yl) 2,2-dimethylpropanethioate,
((2R,3S,4S,5R)-5-(4-amino-2-oxopyrimidin-l(2H)-yl)-4-fluoro-3-hydroxytetrahydrot hiophcn-2-yl)methyI bis(pivaloyloxymethyl) phosphate,
((2R,3Ss4S,5R)-5-(4-amino-2-oxopyrimidin-l(2H)-yl)-4-fluoro-3-hydroxytetrahydrot hiophen-2-yl)methyl bis(2-((2-(benzyloxy)ethyl)disulfanyl)ethyl) phosphate,
((2R;3Ss4S,5R)-5-(4-amino-2-oxopyrimidin-l(2H)-yl)-4-fluoro-3-hydroxytetrahydrot hiophen-2-yl)methyl bis(2-((2-pivaloyloxyethyl)disulfanyl)ethyl) phosphate,
((2RJ3S,4S,5R)-5-(4-amino-2-oxopyrimidin-l(2H)-yl)-4-fluoro-3-hydroxytetrahydrot hiophen-2-yl)methyl bis(2-((2-isobutyroyloxyethyl)disulfanyl)ethyl) phosphate,
S-(2-(((RS)-(((2R,3S,4S,5R)-5-(4-ammo-2-oxopyrimidin-l(2H)-yl)-4-fluoro-3-hydro xytetrahydrothiophen-2-yl)methoxy)(phenoxy)phosphoryl)oxy)ethyl) 2,2-dimethylpropanethioate,
4-amino-l-((2R,3S,4Ss5R)-3-fluoro-4-hydroxy-5-((((RS)-2-oxido-4H-benzo[d][l,3,2] dioxaphosphinin-2-yl)oxy)methyl)tetrahydrothiophen-2-yl)pyrimidin-2(IH)-one,
(2S)-benzyl 2-((t2RS,4aR,6R,7S,7aS)-6~(4-amino-2-oxopyrimidin-l(2H)-yl)-7-fluoro-2-oxidotetrahydro-4 H-tliieno[3,2-d][l,3,2]dioxaphosphinin-2-yl)amino)propanoate, and
(2S)-isopropyl 2-(((2RSs4aR,6R7SJ7aS)-6-(4-amino-2-oxopyrimidin-l(2H)-yl)-7-fluoro-2-oxidotetrahydro-4 H-thieno[3,2-d][l,3,2]dioxaphosphinin-2-yl)amino)propanoate; or the salt thereof.
7. A pharmaceutical composition comprising the thionucleoside derivative or the salt thereof according to any one of claims 1 to 6.

8. The pharmaceutical composition according to claim 7, for use in the treatment of a tumor.
9. The pharmaceutical composition according to claim 7, for use in the treatment of lung ■ cancer, esophageal cancer, stomach cancer, colon cancer, rectal cancer, pancreatic cancer,
breast cancer, renal cancer, bladder cancer, uterine cancer, osteosarcoma, melanoma, leukemia, multiple myeloma, or malignant lymphoma.