Matrix metalloproteinases (MMPs) expression plays a critical role in extracellular matrix deposition. Although several pieces of evidence have so far indicated that gelatinase contributes to the development of pulmonary fibrosis, the role of collagenase remains uncertain. In this study, we attempted to determine the role of collagenase using a bleomycin-induced pulmonary fibrosis model. Bleomycin was instilled into mice intratracheally. Bronchoalveolar lavage fluid (BAL) specimens were analyzed for gelatin and casein zymography, as well as by immunoblotting. The histology of the lungs and hydroxyproline contents were also assessed. MMPs inhibitor, CGS27023A, was simultaneously orally administered. Collagenases were induced in BAL fluids after bleomycin administration based on the data of zymography and immunohistochemistry. The co-administration of MMPs inhibitor, CGS27023A, with bleomycin resulted in worsening pulmonary fibrosis with inhibition of collagenase. The worsening of pulmonary fibrosis was mainly induced by CGS27023A administration in the late phase of bleomycin-induced pulmonary fibrosis development, but not in the early phase. The present data indicated that collagenase plays an anti-fibrotic role in the bleomycin-induced pulmonary fibrosis model. Collagenase has a greater effect on fibrosis phase than inflammatory phase in the bleomycin-induced pulmonary fibrosis in the mice.