A 79-year-old woman visited our hospital because of lower abdominal discomfort. She had been taking lansoprazole for gastroesophageal reflux disease during the past three months. Abdominal CT showed thickening of the sigmoid colon wall. Abdominal ultrasound showed thickening of the sigmoid colon wall with unclear stratification. Colonoscopy revealed a longitudinal ulcer of the sigmoid colon. Histopathological assessment of biopsy specimens showed deposition of a collagen band beneath the epithelium and inflammatory cell infiltration in the lamina propria. These findings were consistent with collagenous colitis. After discontinuation of lansoprazole, her symptom improved and thickening of the sigmoid colon wall resolved.