Provided is a novel compound that has an excellent β-lactamase inhibitory effect. More specifically, provided is a compound represented by formula (1a), (1b) or (11) having an excellent β-lactamase inhibitory effect or a pharmaceutically acceptable salt thereof. By using this compound either in combination with a β-lactam drug or alone, a useful preventive or therapeutic agent for bacterial infections is provided. Also provided are useful preventive or therapeutic agents for treating various diseases with the combined use of the aforesaid compound and β-lactam drugs.