A gamma-ray imager called Compton camera is expected as a new medical instrument for nuclear medicine. It has several advantages over the conventional techniques such as positron emission tomography and single photon emission tomography. Especially, the Compton camera can expand the range of diagnosis and therapy because various gamma-ray energies can be measured without changing system configuration. Many studies have been made on the Compton camera, however, its medical application is still challenging because in clinical environments it is very difficult to image targets due to large background activities. We will establish the imaging techniques and demonstrate the imaging capabilities of the advanced Compton camera. We constructed the prototype system with the gaseous chamber and the scintillation counter. We performed phantom experiments with a 131I gamma-ray source, assuming clinical environments. We evaluated the contrast resolution and found the limitations on the concentration ratio and the magnitude of statistics for imaging hotspots in the large background activity. We performed the Compton tomography in the only three-direction scan and obtained an axial image. We also demonstrated the fusion image with X-ray CT for clinical application.