This study proposes a new method to reconstruct 3D radiation distribution considering objects in measurement environment. A mobile robot equipped with a gamma-ray detector and a 3D LiDAR is used. The gamma-ray CT is applied for image reconstruction. The mobile robot observes radiation at different points and localizes the position of the robot. In observing process, the robot makes environment map which is made from point clouds at multiple observing points. By using this map made from point clouds, precise reconstruction area is set and reconstruction is conducted in the designated area. Experimental results show the effectiveness of the proposed method.