Provided is an antibacterial activity potentiator for glycopeptide antibacterial drugs, which is used against glycopeptide antibacterial drug-resistant bacteria such as vancomycin-resistant bacteria. The antibacterial activity potentiator for glycopeptide antibacterial drugs and/or aminoglycoside antibacterial drugs, which is used against glycopeptide antibacterial drug-resistant bacteria, contains a dimeric sesquiterpene thioalkaloid.