Quenchbody (Q-body) is a fluorescent biosensor in which a fluorescent dye is tagged near the antigen binding site of an antibody. The fluorescence of the dye is quenched by the tryptophan residues present in the variable region of the antibody, and is recovered when the antigen binds. Q-bodies have been prepared using recombinant DNA technology by introducing one or more tag sequence(s) at either the N-terminal of the Fab or the single chain variable region fragment of the antibody, and labeling the tag with a fluorescent dye. However, preparation of recombinant antibody fragments is time-consuming and the performance of the Q-body is unpredictable. Here we report an antibody-binding quenching probe made from protein M from Mycoplasma genitalium that can transform the IgG antibody into an immunosensor. By using bacterially expressed and purified protein M and labeling the C-terminal cysteine-containing tag, we prepared a TAMRA-labeled PM Q-probe. When the Q-probe was incubated with Fab or IgG recognizing the bone Gla protein, the fluorescence of the probe was quenched and subsequently recovered by the adding of antigens in a dose-dependent manner. We also succeeded in detecting several small biomarkers with nanomolar sensitivity, including thyroxine extracted from human serum. The clone found to be suitable for the detection of cortisol was confirmed to work as a recombinant Q-body as well, which also worked in 50% human serum. The results suggest that the Q-probe can quickly convert an IgG to a biosensor, which will be useful in rapid diagnosis of small biomarkers.