A 65-year-old man was admitted to our hospital because of mild dyspnea. A chest roentgenogram showed diffuse reticulonodular shadows in both lung fields. The concentration of CA19-9 in serum was high. Pancreatic cancer and other diseases were considered as causes, but no definitive diagnosis could be made. An open-lung biopsy was done, and examination of a specimen resulted in the diagnosis of idiopathic interstitial pneumonia (chronic type). Immunohistochemical staining with anti-CA19-9 antibody was positive. The lumens of microscopic honeycomb structures and fibrotic areas were covered with flattened and cuboidal metaplastic epithelial cells, which stained positively for anti-CA19-9 antibody.