It is known that Drug Delivery System (DDS) is useful to remedy against tumors for the reduction of side effects and the effective dosage. However the shape, in particular, the size of drug (medicine) is empirically decided in the present stage, which will be related to a question how much medicine should be dosed. Taking a particular reaction of tumor tissues called the EPR effect into consideration, we try to mathematically describe the behavior (dynamics) of drug in blood vessel by applying several techniques used in mathematics and physics. In this paper, we estimate the configuration of drug which is most effective to remedy for tumors under various conditions. As a result, this model and its simulation will be useful to design the drug in nano-level.