The β-lactamase inhibitor, sulbactam, was tested for β-lactamase inhibitory activity in Pseudomonas aeruginosa cells producing various levels of both the MexAB-OprM efflux pump and β-lactamase. We found that sulbactam lowered the MICs of cefoperazone and piperacillin by inhibiting the β-lactamase 8-fold in the cell producing a constitutively high level of AmpC-type β-lactamase and a wild-type level of MexAB-OprM pump compared with that without sulbactam. The MICs of cefoperazone and piperacillin in the cell producing a constitutively high level of both the efflux pump and β-lactamase under the presence of sulbactam were 8 and 4 times, respectively, lower than that without sulbactam. The MICs of sulbactam in the cell producing a constitutively high and a wild-type level of the efflux pump were 16- and 8-fold higher, respectively, than that in the mutant lacking the efflux pump. We concluded that sulbactam exerts potent β-lactamase inhibitory activity in the cell producing a high level of efflux pump, in spite of the fact that sulbactam serves as a substrate of the MexAB-OprM pump. Increasing amounts of sulbactam over the weight of β-lactams further strengthen the effect of β-lactam antibiotics.