The instability of diborane gas and its influence on silicon epitaxial film growth are evaluated, for the first time, from the viewpoint of the chemical behavior of boranes in a silicon epitaxial reactor which is mainly made of silicon oxide (quartz) and stainless steel. On the surface of silicon oxide, deposition of a film mainly composed of boron can occur above 500 K. The fluctuation of the diborane gas concentration in the order of ppm in the stainless-steel tube at room temperature intensely influences the reproducibility of the boron incorporation into the silicon epitaxial him grown in a diborane-trichlorosilane-hydrogen system. The mechanism causing the instability of diborane gas is discussed from the viewpoint of thermolysis, adsorption and desorption of boranes.