The monoclonal antibody A7 (Mab A7) against human colonic cancer also reacts with human gastric cancer at a high rate. We produced a conjugate of neocarzinostatin (NCS) with Mab A7 (A7-NCS). The in vitro anticancer effect of A7-NCS on the antigen-positive human gastric cancer cell line MKN45 was stronger than that of free NCS. Nude mice models of peritoneal dissemination were established by the intra-peritoneal inoculation of MKN45. These models were divided into three groups. The anticancer effect observed in the group that received the intra peritoneal injection of A7-NCS was superior to that observed in the group that received the intra-venous injection and the group that received no treatment. In conclusion, the intra-peritoneal injection of A7-NCS was a useful treatment method for the peritoneal dissemination of gastric cancer.