The cosmic MeV gamma-ray observation is a promising diagnostic tool to address the universe. While INTEGRAL and COMPTEL unveiled the MeV gamma-ray sky, the outstanding issue, like the origin of the gamma-ray and positron excesses toward the galactic inner region, remained. Furthermore, the conventional nonlinear imaging with the superposition of the Compton circles and the coded mask aperture system cannot discriminate the background from the signal. To overcome such difficulties, we developed an Electron-Tracking Compton Camera (ETCC), which has a linear imaging system. The balloon experiment on April 2018 in Australia, the so-called SMILE-2+, was carried out, and we observed the galactic diffuse gamma-rays with a significance of 4.3𝜎 in the energy range of 150–600 keV. The gamma-ray flux was consistent with the point-source emission and the annihilation radiation from the positronium observed from INTEGRAL/SPI.