Collagenous colitis (CC) is an illness characterized by chronic diarrhea with possible effects on neoplastic development, but there have been no reports in animals. We therefore tried to establish CC development in a Mongolian gerbil (MG) model by long-term continuous lansoprazole (LPZ) administration and aimed to clarify the relationship between LPZ administration and CC occurrence. We divided 69 gerbils into 6 groups: Helicobacter pylori (Hp)-infected+high-dose LPZ, Hp-infected+low-dose-LPZ, Hp-infected, high-dose-LPZ, low-dose-LPZ, and control. The gerbils were sacrificed and entire colons were excised at experimental weeks 27, 54, and 108. We examined colonic lesions by staining of Swiss-roll intestines pathologically. A total of 3 gerbils had CC-like lesions in the proximal colon. All MGs with CC-like lesions were from LPZ treated groups (3 of 35; 8.6%). The thickened subepithelial collagen band detected in these lesions strongly resembled that of human CC lesions. Immunohistochemical analysis indicated a tendency for more chromogranin A-positive cells in the upper layer of colonic crypt following continuous LPZ administration. In conclusion, we successfully established development of CC-like lesions in an MG model by continuous LPZ administration and determined that the ectopic endocrine cells that were induced by LPZ administration may influence the occurrence of these lesions in the colon.