Abstract: A problem of the present invention is to obtain a preparation which shows markedly enhanced effectiveness and safety by exhibiting high accumulability at diseased areas and having the blood concentration maintained at a constant level without individual differences, and for which the control of the dosage depending on the blood kinetics (blood trough concentration) becomes unnecessary. The invention relates to a polymeric derivative of tacrolimus including a copolymer of a polyethylene glycol segment and a polymeric moiety, in which a carboxy group in a side chain of the copolymer is bonded to an alcoholic hydroxy group of tacrolimus.