PURPOSE: To obtain the high-strength and high-elasticity sheet excellent in workability at the time of obtaining a pyrolytic boron nitride film by a chemical vapor-phase reaction by specifying the film forming temp., film forming rate and crystal structure.

CONSTITUTION: A gaseous boron halide is allowed to react with gaseous ammonia at ≥1800°C and at the film forming rate of ≤20μ/hr by CVD method to deposit a pyrolytic boron nitride (p-BN) film on a substrate, and then the film is separated from the substrate. A sheet with the Young's modulus of the BM layer controlled to ≤ 5 GPa and its thickness to 10-300μ and consisting of a mixed layer of hexagonal boron nitride (h-BN) and a turbulent-structure boron nitride (t-BN) having the interplanar spacing of C faces larger than that of h-BN or a mixed layer of two kinds of t-BN having different interplanar spacings of C faces is obtained by this method.