We have developed an Electron Tracking Compton Camera (ETCC) as an MeV gamma-ray telescope in the next generation. Our detector consists of a gaseous time projection chamber and a position sensitive scintillation camera. In order to evaluate the performance of this detector, we constructed a flight model detector as a balloon experiment for the observation of diffuse cosmic gamma rays and atmospheric gamma rays. The balloon launched on September 1 in 2006. The balloon reached to 35 km at altitude, and the level flight continued during 4.0 hours. We succeeded in the detection of about 200 downward gamma rays in the energy range of 100 keV - 1 MeV during the 3.5 hours level flight (live time 3.0 hours). The detected photon number was consistent to the simulated one. Also, we measured the dependence of the gamma-ray flux on the zenith angle between 0° and 90°. Here we report on the detailed performance of the ETCC in this flight.