As the next generation of sub-MeV/MeV gamma-ray explorer for astronomy, we have developed an electron-tracking Compton camera (ETCC) which consists of combination of a gaseous ray-tracing chamber as a Compton-scattering target and position sensitive scintillator arrays as a scattered gamma-ray absorber. Our detector has full ray-tracing capability for Compton scattering reconstruction, the capability has two significant benefits for Compton gamma-ray imager, one is efficient background rejection without reduction of effective area for gamma rays, and another one is clear imaging. We had launched a balloon-borne experiment of a small size ETCC in 2006, and succeeded for observation of the spectra of diffuse cosmic and atmospheric gamma rays. However, obtaining the proof of imaging capability by observation of a bright celestial object such as the Crab nebula is strongly required to propose an all-sky survey satellite. Aiming to the proof of clear imaging, we are planning a next balloon flight and we have constructed a 30 cm-cube size ETCC which has about 100 times higher sensitivity than the previous flight detector. This sensitivity enables us to detect the Crab nebula with 5 sigma level for several hours observation. We report imaging demonstrations at the ground and performance tests of gamma-ray detection efficiency, energy resolution, and imaging resolution in range of 150 keV to 1 MeV gamma-ray band.