The present invention provides a therapeutic agent for anaerobic diseases such as solid tumor, comprising in combination a pharmaceutical composition for the treatment of an anaerobic disease containing the transformed anaerobic microorganism as an active component and a pharmaceutical composition containing as an active component an anaerobic microorganism colonization and growth enhancer for enhancing the specific colonization and proliferation of the anaerobic microorganism at an anaerobic disease site. Furthermore, the present invention provides to an anaerobic microorganism colonization and growth enhancer for enhancing colonization and growth of the transformed anaerobic microorganism at a disease site that is in an anaerobic environment.