A waveguide grating on a transparent substrate can serve as an optical notch filter owing to guided-mode resonance. The filtering wavelength is highly sensitive to an incidence angle. Potential of a two-dimensional grating for an application to three-dimensional angle-fluctuation sensing was investigated. Spectral variations due to the angle fluctuations were theoretically discussed and simulated using the rigorous coupled wave analysis. Incidence and azimuth angles (θ, ψ) are measured from resonance wavelengths while a polarization angle φ is detected from a ratio in transmittance at the resonance wavelengths. Sensitivities by a designed example were estimated to be 5 and 10 nm/° and 0.3 dB/° for θ, ψ, and φ, respectively.