A 65-year-old man is presented here with a huge mass of 13 cm in diameter in the left upper abdomen. Histopathologic assessment of endoscopic forceps biopsy revealed a c-kit positive gastrointestinal storomal tumor (GIST) of the stomach. Abdominal computed tomography (CT) showed a direct invasion to the pancreas. Imatinib mesilate was administered as neoadjuvant therapy according to the NCCN Guidelines. Imatinib mesilate therapy was stopped within 2 weeks because of adverse events such as Grade 2 of facial edema and dizziness. However, no hematological adverse event was shown. After three months of treatment (relative dose intensity was 87.5%), CT revealed a reduction in tumor diameter of 35.6% and showed no longer a direct invasion to the pancreas. The radical operation was considered feasible and partial gastrectomy was performed. The tumor was well encapsulated and radical surgery was possible without rupture. Adjuvant therapy was not performed. The patient has now been in good health without a recurrence for three months after the surgery.