Disclosed is a drug delivery system for delivering a drug at a sustained constant rate for a long period, which can be transplanted into an affected part safely and in a simple manner and can deliver a drug to the affected part for a long period. Specifically disclosed is a sustained drag delivery system in which an implant is implanted into a body, wherein the implant is a PEG capsule comprising a box-shaped PEG and a porous PEG sheet, and wherein the box-shaped PEG has, encapsulated therein, a solution of a therapeutic drug, a collagen impregnated with the therapeutic drug, or a PEG pellet including the collagen impregnated with the therapeutic drug, and the PEG capsule has such a structure that the box-shaped PEG is covered with the porous PEG sheet so that the drug contained in the PEG capsule can be released through the porous PEG sheet in a sustained manner.
