A new method to measure high-energy gamma-rays from astronomical objects has been developed. In this method, a commercial cargo airplane was used in order to work at an altitude near to the air shower maximum. gamma-rays with energies greater than 40 GeV have been measured by a lead-glass based electron telescope. The angular resolution was estimated to be approximately 1-degrees. The first experiment has been carried out on a flight between Narita, Japan and Sydney, Australia on June 7, 1989 (JST). We have observed a possible structure of gamma-ray emission from the Galactic plane.