Technology for measuring and identifying the positions and distributions of radioactive substances is important for decommissioning work sites at nuclear power stations. A three-dimensional (3D) image reconstruction method that locates radioactive substances by integrating structure-from-motion (SfM) with a Compton camera (a type of gamma-ray imager) has been developed. From the photographs captured while freely moving in an experimental environment, a 3D structural model of the experimental environment was created. By projecting the radioactive substance image acquired by the Compton camera on the 3D structural model, the positions of the radioactive substance were visualized in 3D space. In a demonstration study, the 137Cs-radiation source was successfully visualized in the experimental environment captured by the freely moving cameras. In addition, how the imaging accuracy is affected by uncertainty in the self-localization of the Compton camera processed by SfM, and by positional uncertainty in the gamma-ray incidence determined by the sensors of the Compton camera was investigated. The created map depicts the positions of radioactive substances inside radiation work environments, such as decommissioning work sites at nuclear power stations.