We prepared the poly(succinimide) (hereafter abbreviated as PSI) microcapsules with antitumor agent showing a pH-responsive release. PSI microcapsules enclosing Irinotecan hydrochloride (CPT-11) were prepared by solvent evaporation method via O/O emulsion system. The obtained PSI microcapsules had a smooth surface and were spherical with diameter of approximately 30 urn. CPT-11 was successfully microencapsulated with high entrapment efficiency of above 90 %. From the results of in vitro release test in an aqueous media with variety of pH, CPT-11 was constantly released for 1 week with no initial burst at pH 5. On the other hand, it was clearly observed the pH-dependency of release rate. That is, the release rate was increased at the higher pH in aqueous media. From SEM observations of internal structure of PSI microcapsules after incubation in aqueous media, it was found that the hole was observed in center of microcapsules.