The present invention provides an active targeting polymer micelle encapsulating a drug, preventing an inappropriate release of a drug which may damage a normal cell. A polymer micelle 100 includes a backbone polymer unit 10 that has a target binding site 11 and a backbone polymer unit 20 that has a drug 14 and is free from the target binding site 11, such polymer units 10 and 20 being disposed in a radial arrangement in a state where the target binding site 11 is directed outward and the drug 14 is directed inward, in which: i) when the micelle is bound to a target 40 while maintaining the radial arrangement, the micelle is taken up into a cell 50 supplying the target 40 through endocytosis, and the drug 14 is released into the cell 50 by collapse of the radial arrangement in the cell 50； and ii) when the radial arrangement collapses in blood 60 before the micelle is bound to a target 40, the unit 20 is excreted through metabolism, to thereby prevent a normal cell from being damaged by the drug 14.
