A volumetric representation of three-dimensional (3D) space is important for underwater navigation and mapping. In this study, a systematic 3D reconstruction scheme using a wide-beam imaging sonar is proposed. For efficient 3D reconstruction, an occupancy grid map with an octree structure is employed and a sensor model considering the sensing characteristics of the imaging sonar is introduced. In order to investigate the feasibility of the proposed reconstruction approach, an experiment has been carried out in a water tank.