The present invention has an object of providing a drug carrier capable of controlling in vivo pharmacokinetics. The present invention is directed to a drug carrier comprising a molecular assembly having a drug incorporated therein, and the above object can be achieved by a part of the amphiphilic molecules included in the molecular assembly being released from the molecular assembly by an external environmental change. The present invention utilizes a phenomenon that the hydrophilic-hydrophobic balance of the amphiphilic molecules is shifted toward hydrophilicity by an external environmental change and thus the amphiphilic molecules are freed from the molecular assembly.
