Abstract: The present invention relates to a novel pyrrolobenzodiazepine dimer compound or a pharmaceutically acceptable salt thereof, a ligand-drug conjugate compound thereof, or a composition containing the same and therapeutic use of the same as an anticancer agent. The pyrrolobenzodiazepine dimer compound according to the present invention exhibits anticancer activity equivalent to or superior to that of existing anticancer agents when being applied to a ligand-drug conjugate as a drug and administered as well as exhibits low activity and greatly diminished toxicity in the free toxin form and thus has a significantly improved therapeutic index. Hence, the pyrrolobenzodiazepine dimer compound is highly industrially applicable in that targeting of proliferative diseases such as cancer is possible, specific treatment of the proliferative diseases is possible, the drug efficacy can be maximized, and the expression of side effects can be minimized.