In dense urban environments, aerial image acquisition often suffers from occlusions and redundant data due to the lack of quantitative evaluation tools at the flight-planning stage. To address this issue, this study develops a flight-planning support system that enables pre-acquisition visibility analysis for both terrain and building surfaces using existing 3D city models. The system performs ray-casting simulations based on user-defined flight parameters to quantify and visualize occluded and visible regions before flight, allowing planners to evaluate data quality and optimize image acquisition efficiency. Experiments were conducted using real flight plans with two representative aerial cameras: the Leica CityMapper-2 for multi-directional texture mapping and the Vexcel UltraCam Eagle 4.1 for nadir-based topographic mapping. The results show that the system effectively visualizes occlusions on roofs and walls, predicts building lean in nadir imagery, and assesses the influence of overlap ratios on ground visibility. These analyses enable users to design more cost-effective and geometrically consistent flight plans by identifying redundant overlaps and ensuring sufficient coverage for DSM and true-orthophoto generation. The proposed framework provides a quantitative and objective approach to improving the transparency and reliability of aerial survey planning, and it offers a foundation for integrating visibility simulation with subsequent photogrammetric workflows such as surface reconstruction and texture mapping.