The present method of manufacturing a GaN-based film includes the steps of preparing a composite substrate including a support substrate dissoluble in hydrofluoric acid and a single crystal film arranged on a side of a main surface of the support substrate, a coefficient of thermal expansion in the main surface of the support substrate being more than 0.8 time and less than 1.2 times as high as a coefficient of thermal expansion of GaN crystal, forming a GaN-based film on a main surface of the single crystal film arranged on the side of the main surface of the support substrate, and removing the support substrate by dissolving the support substrate in hydrofluoric acid. Thus, the method of manufacturing a GaN-based film capable of efficiently obtaining a GaN-based film having a large main surface area, less warpage, and good crystallinity, as well as a composite substrate used therefor are provided.