A 76-year-old man was admitted to our hospital because of tarry motions. Endoscopic findings showed an ulcer on a large submucosal tumor in the stomach. Abodminal CT scan showed a protruding lesion of approximately 13 cm at the lumen of the gastric body. FDG-PET imaging revealed FDG uptake in the gastric body and abdominal cavity. We diagnosed it as GIST with peritoneal dissemination clinically, and treatment with 300 mg of imatinib mesylate was started in December 2006. The main tumor was reduced(reduction rate of 27%)and FDG-PET imaging revealed a decrease in FDG uptake in the main tumor and all disseminated tumors after 5 months of treatment. However, the drug was discontinued for arthritis(grade 3). Partial gastrectomy with sampling peritoneal nodules was performed in June 2007. The present case suggests that low-dose chemotherapy with imatinib mesylate may be useful as a preoperative therapy for a minimal surgery.