An object of the invention is to obtain a preparation which shows enhanced effectiveness and safety compared to tacrolimus, has the concentration in blood maintained at a constant level without individual differences, whereby the control of the amount of administration depending on the blood kinetics (blood trough concentration) becomes unnecessary. The present invention relates to a polymeric derivative of tacrolimus, in which a polyethylene glycol segment and an alcoholic hydroxy group of tacrolimus are bonded to side-chain carboxy groups of a polyaspartic acid derivative.
