An object of the present invention is to provide an antitumor agent and/or a marker for tumor detection with low invasiveness, few side effects, and high target specificity. Through the use of a recombinant obligate anaerobic gram-positive bacterium as an active ingredient containing a nucleic acid encoding a secretory fusion protein comprising a signal peptide, a low-molecular-weight single-chain antibody, and a functional peptide, an antitumor agent that enables delivery of a functional peptide to a target cell and/or a marker for tumor detection that enables monitoring of therapeutic effects over time are provided.
