A cavity-resonator-integrated guided-mode resonance filter (CRIGF) consists of a grating coupler (GC) integrated in a waveguide cavity resonator on a transparent substrate, and shows a narrowband reflection spectrum for an incident free-space wave of a small diameter. CRIGF of the waveguide cavity formed by two distributed Bragg reflectors (DBRs) of different reflectances is discussed. Such a CRIGF can act as not only a wavelength-selective reflector but also an input waveguide coupler. Integration of such CRIGFs for different wavelengths in a waveguide is proposed to give an array of external mirrors and a wavelength division multiplexer for constructing a compact multi-wavelength light source. Four CRIGFs of 10-µm-size aperture with a wavelength spacing of 15 nm were fabricated and characterized. CRIGF integrated on a reflective layer via an optical buffer layer is also discussed. Optimization of reflective and buffer layers is theoretically investigated to maximize a waveguide-coupled lasing power.