Abstract: The present invention relates to a unit drug conjugate wherein a binding group capable of binding to another unit drug conjugate is additionally linked to a unit drug conjugate in which a targeting substance, which binds specifically to target cells, and a drug, are linked together. When the unit drug conjugates according to the present invention are sequentially administered in vivo, complexes form a cluster by in vivo crosslinking, and the cluster promotes endocytosis of the drug conjugate, thereby significantly enhancing cellular internalization of the drug included in the drug conjugate.