A method for preparing a compound represented by general formula I: [wherein R 1 and R 2 represent independently of each other a protecting group for hydroxy group, R 3 and R 4 represent independently of each other a hydrogen atom or a C 1-6 alkyl group which may be optionally substituted with one or more substituents, each R 5 represents independently a hydrogen atom, or a C 1-6 alkyl group which may be optionally substituted with one or more substituents, each R 6 represents independently a hydrogen atom, or a C 1-6 alkyl group which may be optionally substituted with one or more substituents, m is an integer of 1 to 3, and a cycle A represents a five to seven membered unsaturated heterocyclic group which may be optionally substituted with one or more substituents selected from the group consisting of a C 1-6 alkyl group which may be optionally substituted with one or more substituents, a C 2-6 alkenyl group which may be optionally substituted with one or more substituents, a C 2-6 alkynyl group which may be optionally substituted with one or more substituents, an amino group which may be optionally substituted with one or more substituents, a hydroxy group which may be optionally substituted with one or more substituents, an oxo group, a thioxo group, and a halogen atom, and the unsaturated heterocyclic group may be further fused with another cycle to form a cycle A.] or salts thereof, said method comprising a step of reacting a compound represented by formula II: [wherein, each of R 1, R 2, R 3, R 4, R 5, R 6 and m is the same as defined in the above-mentioned general formula I, a cycle A' represents a five to seven membered unsaturated heterocyclic group which may be optionally substituted with one or more substituents selected from the group consisting of a C 1-6 alkyl group which may be optionally substituted with one or more substituents, a C 2-6 alkenyl group which may be optionally substituted with one or more substituents, a C 2-6 alkynyl group which may be optionally substituted with one or more substituents, an amino group which may be optionally substituted with one or more substituents, a hydroxy group which may be optionally substituted with one or more substituents, an oxo group, a thioxo group, and a halogen atom, and the unsaturated heterocyclic group may be further fused with another cycle to form a cycle A'.] with a reducing agent to cleave an oxazolidine ring fused to a cycle A'. R 1 and R 2 represent independently of each other a protecting group for hydroxy group, R 3 and R 4 represent independently of each other a hydrogen atom or a C 1-6 alkyl group which may be optionally substituted with one or more substituents, each R 5 represents independently a hydrogen atom, or a C 1-6 alkyl group which may be optionally substituted with one or more substituents, each R 6 represents independently a hydrogen atom, or a C 1-6 alkyl group which may be optionally substituted with one or more substituents, m is an integer of 1 to 3, and a cycle A represents a five to seven membered unsaturated heterocyclic group which may be optionally substituted with one or more substituents selected from the group consisting of a C 1-6 alkyl group which may be optionally substituted with one or more substituents, a C 2-6 alkenyl group which may be optionally substituted with one or more substituents, a C 2-6 alkynyl group which may be optionally substituted with one or more substituents, an amino group which may be optionally substituted with one or more substituents, a hydroxy group which may be optionally substituted with one or more substituents, an oxo group, a thioxo group, and a halogen atom, and the unsaturated heterocyclic group may be further fused with another cycle to form a cycle A.] or salts thereof, said method comprising a step of reacting a compound represented by formula II: [wherein, each of R 1, R 2, R 3, R 4, R 5, R 6 and m is the same as defined in the above-mentioned general formula I, a cycle A' represents a five to seven membered unsaturated heterocyclic group which may be optionally substituted with one or more substituents selected from the group consisting of a C 1-6 alkyl group which may be optionally substituted with one or more substituents, a C 2-6 alkenyl group which may be optionally substituted with one or more substituents, a C 2-6 alkynyl group which may be optionally substituted with one or more substituents, an amino group which may be optionally substituted with one or more substituents, a hydroxy group which may be optionally substituted with one or more substituents, an oxo group, a thioxo group, and a halogen atom, and the unsaturated heterocyclic group may be further fused with another cycle to form a cycle A'.]