We report a case of autoimmune gastritis (AIG) in which gastric mucosal atrophy improved with Helicobacter pylori eradication. Based on endoscopic findings (advanced gastric atrophy with vascular visibility and diffuse redness in remnant oxyntic mucosa), a woman in her 40s was suspected of having AIG coexisting with an active H. pylori infection. This was confirmed by a positive anti-parietal cell antibody (PCA, 1:160), an elevated serum gastrin level (638 pg/mL), and positive anti-H. pylori antibody (Hp Ab, 15.5 U/mL) and H. pylori stool antigen tests. Seven months after eradication, reduced vascular visibility and disappearance of diffuse redness on endoscopy and reduced PCA (1:40) and Hp Ab (5.1 U/mL) titers were observed, although histopathological findings (basal-predominant lymphocytic infiltration, destruction of parietal and chief cells, pseudopyloric metaplasia, and enterochromaffin-like cell hyperplasia) were consistent with AIG. Endoscopy 26 months after eradication showed further improvement in atrophic findings in the gastric corpus and histopathological recovery of parietal and chief cells in fundic glands. Serum gastrin levels returned to normal (64 pg/mL), and the PCA titer fell further (1:20).