Provided are an optically active diazabicyclooctane derivative defined by formula (F) below, which is useful as a pharmaceutical intermediate for ²-lactamase inhibitor, and a process for preparing the same. In formula (F) above, R 1 represents CO 2 R, CO 2 M, or CONH 2 , wherein R represents a methyl group, a tert-butyl group, an allyl group, a benzyl group, or a 2,5-dioxopyrrolidin-1-yl group, and M represents a hydrogen atom, an inorganic cation, or an organic cation; and R 2 represents a benzyl group or an allyl group.