Due to the small size of Micro Aerial Vehicles, they are well suited for various indoor applications, especially search/rescue operations. Most of these operations are performed in unknown 3D environments. Real-time map construction and vehicle’s localization are essential tasks. Various approaches provide solutions for 3D-map representation. However, these approaches require expensive embedded systems to afford high-processing memory/computational costs. Because of its exposure to risks, the MAV should be equipped with a low-cost navigation system. The principal aim of map construction herein is to facilitate the navigation task. Thus, constructing a massive 3D map is not required. Consequently, this paper proposes an efficient real-time 3D SLAM. The proposed method avoids the 3D-map representation of each region of the trajectory. Alternatively, it divides the environment along the trajectory into several 2D maps with a single 2D map in every region at the height of the MAV, as well as a transient region between each of the two constructed maps to enable connecting neighboring maps.