Identification method for a hidden or shielded gamma-ray source using a vehicle-mounted 4pi gamma-ray imager was proposed. Since 4pi gamma-ray imaging provides direction of incident gamma-rays into the imager and its intensity, a position and an activity of a gamma-ray source can be estimated by several 4pi gamma images obtained by the imager moving around the source in combination with locations and orientation data of the imager. Using a 3D pixel array CdTe detector as a prototype of 4pi Compton gamma-ray imager, both the position and activity of a single 137 Cs point source was estimated quantitatively in 3D voxel space by 4pi Compton gamma-ray images measured at three positions. In addition, 4pi gamma images were obtained by off-line processing of data acquired during the movement of the vehicle-mounted imager with processing time less than measurement time.