To establish imaging spectroscopy of cosmic gamma-rays from a few hundreds of keV to a few tens MeV, we developed an electron-tracking Compton camera (ETCC). The ETCC consists of a time projection chamber (TPC) and pixelated scintillator arrays (PSAs). The ETCC is superior to conventional gamma-ray imaging detectors of this energy band in that the arrival direction of an incident gamma-ray is firmly determined at one point and realizes high noise rejection efficiency. We performed a campaign to demonstrate the gamma-ray imaging performance of the ETCC at balloon altitude via the sub-MeV gamma-ray imaging loaded-on-balloon experiment 2+ (SMILE-2+). The balloon was launched on April 7, 2018, at 6:26 ACST (UTC +9:30) from Alice Springs, Australia. We performed a level flight for 26 hours at an altitude of 39.6 km. The main observation targets were the Galactic Center region and the Crab Nebula and we succeeded in observing them without any critical problems. The configuration of the flight model ETCC and the housekeeping data are described in detail.