When a film such as an atomic layer deposition (ALD) film is formed on a surface of a wafer, there is a problem of a supply amount of a source gas supplied to a film forming apparatus. When the supply amount of the source gas is too small, the source gas for uniformly forming the film becomes insufficient, so that the uniformity of the film is deteriorated. On the other hand, when the supply amount of the source gas is too large, an unnecessary source gas that does not contribute to the formation of the film is wastefully discharged. In addition, when the supply amount of the source gas is too large, a pipe located downstream of the film forming apparatus is often clogged with by-products, resulting in that a maintenance period of a semiconductor manufacturing system becomes short. Also, the supply amount of the source gas needed for forming the film varies depending on a surface area of the wafer. Accordingly, in the case where the supply amount of the source gas is constant, the source gas can be excessive when the wafer surface is flat, and the source gas can be insufficient when the wafer surface has irregularities.