The reliability of the range sensor measurement decreases as the measured length increases. To deal with this problem, we propose a lightweight 3D exploration algorithm for UAVs using target area decomposition based on the reliable sensor range. This paper presents a simple path-search rule and the path candidate generation method based on the sector decomposition. The proposed path-search rule is designed to map the lower sector with priority. The sector is determined by the reliable sensor range, and the centroid of sectors are selected as a path candidate. A* path planner is used for the local path planning and reachability check. The proposed method is validated by conducting the experiment in the GAZEBO simulator.