A compound represented by formula: [wherein R 1 and R 2, which may be the same or different, are each a hydrogen atom, formula -(CR 4 R 4 ')-O-CO-R 5 or -(CR 6 R 6 ')-O-CO-O-R 7, or the following formula (IIa) or (IIb): R 3 is a hydrogen atom, formula - n-H, -CO-O-(CR 9 R 9 ')-O-CO-R 10 or -CO-O-(CR 9 R 9 ')-O-CO-O-R 11, or the following formula: wherein R 4 and R 4 ', which may be the same or different, are each a hydrogen atom or a C 1-6 alkyl group; R 5 is a C 1-10 alkyl group, a C 3-8 cycloalkyl group (the C 3-8 cycloalkyl group being optionally substituted with one to three C 1-6 alkyl groups), an adamantyl group (the adamantyl group being optionally substituted with one to three C 1-6 alkyl groups), or a phenyl group (the phenyl group being optionally substituted with one to three groups selected from a halogen atom and a C 1-6 alkyl group); R 6 and R 6 ', which may be the same or different, are each a hydrogen atom or a C 1-6 alkyl group; R 7 is a C 1-10 alkyl group, a C 3-8 cycloalkyl group (the C 3-8 cycloalkyl group being optionally substituted with one to three C 1-6 alkyl groups), an adamantyl group (the adamantyl group being optionally substituted with one to three C 1-6 alkyl groups), or an aryl group (the aryl group being optionally substituted with one to three groups selected from a halogen atom and a C 1-6 alkyl group); R 8 is a C 1-6 alkyl group or a phenyl group; R 9 and R 9 ', which may be the same or different, are each a hydrogen atom or a C 1-6 alkyl group; R 10 is a C 1-10 alkyl group, a C 3-8 cycloalkyl group (the C 3-8 cycloalkyl group being optionally substituted with one to three C 1-6 alkyl groups), an adamantyl group (the adamantyl group being optionally substituted with one to three C 1-6 alkyl groups), or a phenyl group (the phenyl group being optionally substituted with one to three groups selected from a halogen atom and a C 1-6 alkyl group); R 11 is a C 1-10 alkyl group, a C 3-8 cycloalkyl group (the C 3-8 cycloalkyl group being optionally substituted with one to three C 1-6 alkyl groups), an adamantyl group (the adamantyl group being optionally substituted with one to three C 1-6 alkyl groups), or an aryl group (the aryl group being optionally substituted with one to three groups selected from a halogen atom and a C 1-6 alkyl group); R 12 is a C 1-6 alkyl group or a phenyl group; AA is an aminoacyl group; and n is an integer of 1 to 3, provided that a compound in which R 1, R 2 and R 3 are each a hydrogen atom is excluded] or a pharmaceutically acceptable salt thereof. wherein R 4 and R 4 ', which may be the same or different, are each a hydrogen atom or a C 1-6 alkyl group; R 5 is a C 1-10 alkyl group, a C 3-8 cycloalkyl group (the C 3-8 cycloalkyl group being optionally substituted with one to three C 1-6 alkyl groups), an adamantyl group (the adamantyl group being optionally substituted with one to three C 1-6 alkyl groups), or a phenyl group (the phenyl group being optionally substituted with one to three groups selected from a halogen atom and a C 1-6 alkyl group); R 6 and R 6 ', which may be the same or different, are each a hydrogen atom or a C 1-6 alkyl group; R 7 is a C 1-10 alkyl group, a C 3-8 cycloalkyl group (the C 3-8 cycloalkyl group being optionally substituted with one to three C 1-6 alkyl groups), an adamantyl group (the adamantyl group being optionally substituted with one to three C 1-6 alkyl groups), or an aryl group (the aryl group being optionally substituted with one to three groups selected from a halogen atom and a C 1-6 alkyl group); R 8 is a C 1-6 alkyl group or a phenyl group; R 9 and R 9 ', which may be the same or different, are each a hydrogen atom or a C 1-6 alkyl group; R 10 is a C 1-10 alkyl group, a C 3-8 cycloalkyl group (the C 3-8 cycloalkyl group being optionally substituted with one to three C 1-6 alkyl groups), an adamantyl group (the adamantyl group being optionally substituted with one to three C 1-6 alkyl groups), or a phenyl group (the phenyl group being optionally substituted with one to three groups selected from a halogen atom and a C 1-6 alkyl group); R 11 is a C 1-10 alkyl group, a C 3-8 cycloalkyl group (the C 3-8 cycloalkyl group being optionally substituted with one to three C 1-6 alkyl groups), an adamantyl group (the adamantyl group being optionally substituted with one to three C 1-6 alkyl groups), or an aryl group (the aryl group being optionally substituted with one to three groups selected from a halogen atom and a C 1-6 alkyl group); R 12 is a C 1-6 alkyl group or a phenyl group; AA is an aminoacyl group; and n is an integer of 1 to 3,