A process for preparing the compound of the following formula (E): wherein Bn represents a benzyl group, and tBu represents a tert-butyl group, the process including: (a) subjecting the following compound (B) to trifluoroacetylation, wherein tBu represents a tert-butyl group to produce the following compound (C): wherein tBu represents a tert-butyl group, and TFA represents a trifluoroacetyl group; (b) reacting the compound (C) with benzyloxyamine in the presence of a hydroxyl group activating agent to produce the following compound (D): wherein Bn represents a benzyl group, tBu represents a tert-butyl group, and TFA represents a trifluoroacetyl group; and (c) subjecting the compound (D) to detrifluoroacetylation.