Appropriate and robust behavioral control in a noisy environment is important for the survival of animals. Understanding such robust behavioral control has been an attractive subject in neuroscience research. We addressed this question in the fly brain by investigating optomotor response to noisy optical flow. We found that flies have a robust recognition of optical flow directions and activities of motion sensitive neurons can quantitatively explain the robustness by applying signal classification theory. Our model study suggested that the robustness were ascribed to simple image noise eliminating methods used in the image processing—a Gaussian smoothing and contrast enhancement—.