The purpose of the present invention is to provide an anti-tumour agent and/or marker for tumour detection which have low invasiveness, few side effects, and high target specificity. Provided are a marker for tumour detection, which enables treatment effects to be monitored over time, and/or an anti-tumour agent, which can deliver functional peptides to target cells, the marker and the anti-tumour agent having said properties as a result of using a recombinant strict anaerobe gram-positive bacterium as an active ingredient, said bacterium containing a nucleic acid which codes a secretory fused protein comprising a signal peptide, a low molecular weight single-chain antibody, and a functional peptide.