The present invention provides a molecular assembly whose retention time in a target site is adjusted depending on the kind or purpose of a labeling agent or drug encapsulated therein, and a molecular assembly that can suppress the ABC phenomenon and that can be administered more than once within a short span. A molecular assembly comprising: a branched-type amphiphilic block polymer A comprising a branched hydrophilic block comprising sarcosine and a hydrophobic block comprising a polylactic acid chain； and a functional substance F comprising a functional site and a polylactic acid chain, wherein the polylactic acid chain constituting the hydrophobic block of the amphiphilic block polymer A comprises L-lactic acid units, and the polylactic acid chain contained in the functional substance F comprises D-lactic acid units, or the polylactic acid chain constituting the hydrophobic block of the amphiphilic block polymer A comprises D-lactic acid units, and the polylactic acid chain contained in the functional substance F comprises L-lactic acid units.
