Provided are an optically active diazabicyclooctane derivative defined by formula (F) below, which is useful as a pharmaceutical intermediate for β-lactamase inhibitor, and a process for preparing the same. In formula (F) above, R1 represents CO2R, CO2M, or CONH2, 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 R2 represents a benzyl group or an allyl group.