F-18 FDG-PET/CT could be used to evaluate the surveillance of recurrent stomach cancer, but some cases reported as false-positives. The authors found an activated charcoal granuloma from intraperitoneal chemotherapy by using a curative resection and mitomycin C for stomach cancer. A mass behind the right colon that showed on CT 6 months after an operation in a 46-year-old male patient had no progression in size, but 36 months after the operation, an increase was seen on F-18 FDG-PET/CT, and a metastatic tumor was suspected. The tumor was resected by an explorative laparotomy and was diagnosed as being an activated charcoal granuloma based on the histologic finding. Based on this case, we should be reminded of the possibility of a false-positive on analysis of F-18 FDG-PET/CT caused by an activated charcoal granuloma in a patient who has intraperitoneal chemotherapy.