A hyaluronic acid derivative comprising a repeating unit represented by formula: [where R 1, R 2, R 3, and R 4 are independently selected from a hydrogen atom, C 1-6 alkyl, formyl, and C 1-6 alkylcarbonyl; R 5 is a hydrogen atom, formyl, or C 1-6 alkylcarbonyl; X 1 is hydroxy, -O-Q +, C 1-6 alkoxy, -NR 7 R 8, or -NR 9 -Z 1 -Z 2; Q + represents a counter cation; R 6, R 7, R 8, and R 9 are independently selected from a hydrogen atom and C 1-6 alkyl; R a is a hydrogen atom or C 1-6 alkyl, where the alkyl may be independently substituted by one or more groups selected from hydroxy, carboxy, carbamoyl, C 1-6 alkylthio, aryl, and heteroaryl, where the aryl may be substituted with one or more hydroxy groups; Z 1 is C 2-30 alkylene or -(CH 2 CH 2 O) m -CH 2 CH 2 -, where into the alkylene 1 to 5 groups independently selected from -O-,-NR g -, and -S-S- may be inserted, and m is an integer selected from 1 to 100; Z 2 is selected from groups represented by the following formulas: -NR b -Z 3, -NR b -COO-Z 3, -NR b -CO-Z 3, -NR b -CO-NR c -Z 3, -COO-Z 3, -CO-NR c -Z 3, -O-CO-NR c -Z 3, -O-COO-Z 3, -S-Z 3, -CO-Z a -S-Z 3, -O-CO-Z b -S-Z 3, -NR b -CO-Z b -S-Z 3, and -S-S-Z 3; R b and R c are independently selected from a hydrogen atom, C 1-20 alkyl, amino C 2-20 alkyl, and hydroxy C 2-20 alkyl, where into the alkyl moieties of the groups 1 to 3 groups independently selected from -0- and -NR f - may be inserted; R f is independently selected from a hydrogen atom, C 1-12 alkyl, amino C 2-12 alkyl, and hydroxy C 2-12 alkyl, and into the alkyl moieties of the groups 1 to 2 groups independently selected from -O- and -NH- may be inserted; R g is independently selected from a hydrogen atom, C 1-20 alkyl, amino C 2-20 alkyl, or hydroxy C 2-20 alkyl, and into the alkyl moieties of the groups 1 to 3 groups independently selected from -O- and -NH- may be inserted; Z 3 is a steryl group; Z a is C 1-5 alkylene; and Z b is C 2-8 alkylene or C 2-8 alkenylene], wherein if the hyaluronic acid derivative comprises no repeating units represented by formula in which X 1 is-NR 9 -Z 1 -Z 2, then the hyaluronic acid derivative further comprises a repeating unit represented by formula: [where R 1a, R 2a, R 3a, and R 4a are independently selected from a hydrogen atom, C 1-6 alkyl, formyl, and C 1-6 alkylcarbonyl; R 5a is a hydrogen atom, formyl, or C 1 - 6 alkylcarbonyl; and X 2 is -NR 9 -Z 1 -Z 2, where R 9, Z 1, and Z 2 are as defined above]. [where R 1, R 2, R 3, and R 4 are independently selected from a hydrogen atom, C 1-6 alkyl, formyl, and C 1-6 alkylcarbonyl; R 5 is a hydrogen atom, formyl, or C 1-6 alkylcarbonyl; X 1 is hydroxy, -O-Q +, C 1-6 alkoxy, -NR 7 R 8, or -NR 9 -Z 1 -Z 2; Q + represents a counter cation; R 6, R 7, R 8, and R 9 are independently selected from a hydrogen atom and C 1-6 alkyl; R a is a hydrogen atom or C 1-6 alkyl, where the alkyl may be independently substituted by one or more groups selected from hydroxy, carboxy, carbamoyl, C 1-6 alkylthio, aryl, and heteroaryl, where the aryl may be substituted with one or more hydroxy groups; Z 1 is C 2-30 alkylene or -(CH 2 CH 2 O) m -CH 2 CH 2 -, where into the alkylene 1 to 5 groups independently selected from -O-,-NR g -, and -S-S- may be inserted, and m is an integer selected from 1 to 100; Z 2 is selected from groups represented by the following formulas: -NR b -Z 3, -NR b -COO-Z 3, -NR b -CO-Z 3, -NR b -CO-NR c -Z 3, -COO-Z 3, -CO-NR c -Z 3, -O-CO-NR c -Z 3, -O-COO-Z 3, -S-Z 3, -CO-Z a -S-Z 3, -O-CO-Z b -S-Z 3, -NR b -CO-Z b -S-Z 3, and -S-S-Z 3; R b and R c are independently selected from a hydrogen atom, C 1-20 alkyl, amino C 2-20 alkyl, and hydroxy C 2-20 alkyl, where into the alkyl moieties of the groups 1 to 3 groups independently selected from -0- and -NR f - may be inserted; R f is independently selected from a hydrogen atom, C 1-12 alkyl, amino C 2-12 alkyl, and hydroxy C 2-12 alkyl, and into the alkyl moieties of the groups 1 to 2 groups independently selected from -O- and -NH- may be inserted; R g is independently selected from a hydrogen atom, C 1-20 alkyl, amino C 2-20 alkyl, or hydroxy C 2-20 alkyl, and into the alkyl moieties of the groups 1 to 3 groups independently selected from -O- and -NH- may be inserted; Z 3 is a steryl group; Z a is C 1-5 alkylene; and Z b is C 2-8 alkylene or C 2-8 alkenylene], wherein if the hyaluronic acid derivative comprises no repeating units represented by formula in which X 1 is-NR 9 -Z 1 -Z 2, then the hyaluronic acid derivative further comprises a repeating unit represented by formula: [where R 1a, R 2a, R 3a, and R 4a are independently selected from a hydrogen atom, C 1-6 alkyl, formyl, and C 1-6 alkylcarbonyl; R 5a is a hydrogen atom, formyl, or C 1 - 6 alkylcarbonyl; and X 2 is -NR 9 -Z 1 -Z 2, where R 9, Z 1, and Z 2 are as defined above]. [where R 1a, R 2a, R 3a, and R 4a are independently selected from a hydrogen atom, C 1-6 alkyl, formyl, and C 1-6 alkylcarbonyl; R 5a is a hydrogen atom, formyl, or C 1 - 6 alkylcarbonyl; and X 2 is -NR 9 -Z 1 -Z 2, where R 9, Z 1, and Z 2 are as defined above].