In a 64-year-old man, chest CT revealed thickening of the esophagus wall during an examination of pneumonia. First endoscopy showed diffuse gastric redness with loss of regular arrangement of collecting venules, interpreted as gastritis associated with Helicobacter pylori, but histopathology and breath test were negative. One year later, proteinuria and bilateral edema of the lower limbs developed, and systemic light chain amyloidosis (AL) affecting the kidneys and heart was diagnosed. Repeated endoscopy revealed disk-shaped and linear erythema of the stomach with a protrusion similar to a submucosal tumor. Congo red staining with polarized light confirmed amyloid deposition in the gastric biopsy. On narrow-band imaging (NBI), amyloid-related erythema showed a gray-green signal with a multilayered, deeper-appearing distribution, in contrast to the more superficial appearance of erythema associated with vonoprazan and Helicobacter pylori. Histological confirmation by Congo red staining remains the gold standard for diagnosis.