A diazabicyclooctane compound, which is a beta-lactame inhibitor, represented by the following formula (I): wherein A represents Ra(Rb)N— or RcO—; B represents NH or NC1-6 alkyl; C represents benzyl, H or SO3M, wherein M represents H, an inorganic or an organic cation; Ra and Rb represent H, C1-6 alkyl or acyl; Rc represents C1-6 alkyl or a heterocyclyl; A is unsubstituted or substituted with 0 to 4 substituents Fn1, wherein Fn1 represents C1-6 alkyl, O═, or Rg-(CH2)0-3—, wherein Rg represents a heterocyclyl, phenyl, heteroaryl, acyl, RdO2S—, Re(Rf)N—, Re(Rf)NCO—, ReO—, ReOCO— or a protective group, wherein Rd represents C1-6 alkyl or MO—; Re and Rf represent H or C1-6 alkyl, and a heterocycle having at least one nitrogen atom may be formed between Ra and Rb, between Rc and B, or between Re and Rf.