A quenchbody (Q-body) is an immunosensor comprising an antibody fragment containing an antigen-binding site that is site-specifically labeled with a fluorescent dye. The fluorescent dye of a Q-body is quenched in the absence of an antigen; however, its fluorescence recovers in the presence of an antigen, offering simple and rapid systems for antigen detection. In this study, we fused luciferase NanoLuc to a Q-body to construct a new immunosensor termed the "BRET Q-body" that can detect antigens based on the bioluminescence resonance energy transfer (BRET) principle. The resulting BRET Q-bodies for an osteocalcin peptide that emit three different emission colors could detect an antigen without the requirement of an external light source, based on ratiometric detection and color change with two wavelengths for the luciferase and fluorophore. Furthermore, the BRET Q-body produced unexpectedly higher responses up to 12-fold because of the increased BRET efficiency, probably associated with antigen-dependent dye movement. Thus, the BRET Q-body is a useful biosensor as a core of point-of-care tests.