The common marmoset (Callithrix jacchus) is used as a non-human primate laboratory animal. Marmoset wasting syndrome (MWS) is a disease endemic to captive colonies, and the pathogenesis is unclear. In the present study, marmosets with chronic bloody high-viscosity diarrhea, which is a contributing factor to MWS, were evaluated, and inflammation in the colon was found. Calprotectin is a surrogate marker of intestinal inflammation and induces apoptosis. Marmosets with chronic diarrhea exhibited higher levels of fecal calprotectin. Histochemical analyses showed high expression of calprotectin in the extravascular neutrophils and apoptosis in the chronic colitis lesions. No internal microbiological diseases were identified. Although the cause of chronic colitis was not identified, the marmoset could be a useful model of inflammatory bowel disease.