Heterotopic bone formation in the gastrointestinal tract is a rare phenomenon. Most reported cases were associated with benign and malignant neoplasms, except for a case in which heterotopic bone formation was found in a patient with Barrett's esophagus. The exact pathogenesis of the disease has not yet been established. However, most heterotopic bones found in the gastrointestinal tract were associated with mucinproducing tumors of the appendix, colon, and rectum. Inflammation may also play a role in osseous metaplasia in a case with bone formation at the base of an ulcer in Barrett's esophagus. Here, we report on a patient with heterotopic bone formation in normal gastric cardiac mucosa. A 50-year-old female visited our hospital for a routine health examination. She had no gastrointestinal symptoms, and her physical examination, blood test, X-ray, urine, and stool examination results were normal. A 0.3 cm sized polypoid lesion located just below the squamocolumnar junction was observed on upper gastrointestinal endoscopy. A piece of biopsy was taken. Histologically, a lamella bone trabecula and chronic inflammatory cells were observed in the gastric cardiac mucosa. The follow-up endoscopy performed one month later showed no residual lesion.