The occurrence of early gastric carcinoma with invasion to the duodenum is supposed to be very low, although advanced cancers arising in the antrum can often invade the duodenal area. Generally, malignant invasion of the duodenum is difficult to diagnose preoperatively, as spread of gastric cancer to the duodenum is often infiltrative and invades through the submucosal or subserosal layer. We report an unusual case of an intramucosal gastric carcinoma with extensive duodenal invasion that was preoperatively diagnosed by endoscopy.