We report a case of gastro-colic fistula caused by ectopic gastric mucosa developed at transverse colon. Fistula was detected by colonofiberscopy. And fistulous tract was proved by barium enema. Meckel's diverticulum scan finding was similar to that of GI bleeding; e.g. injected radioactivity was secreted into the lumen and moved along the lumen. There was no bleeding. And there was no diverticulum in the colon. Absence of diverticular pouch may explain this unusuaal GI bleeding-like scan finding rather than focal collection of radioactivity, which is typical of ectopic gastric mucosa found in the Meckel's diverticulum. Ectopic gastric mucosa was confirmed by colonfiberscopic biopsy. We suggest GI bleeding-like pictures should be included differential diagnosis of Tc-99m-O4 (ectopic gastric mucosa or Meckel's diverticulum) scan.