Provided are a thin film forming method capable of stably forming a homogeneous film at high productivity and a thin film stack obtained thereby. In the thin film forming method, a first discharge space for forming a functional film on a base material under an atmospheric pressure or a pressure near the atmospheric pressure and a second discharge space for post-treating the base material on which the film is formed in the first discharge space are provided. The first discharge space is composed of a pair of roll electrodes and used in a film forming step of forming the functional film on the base material by supplying a mixed gas (1) containing a discharge gas and a thin film forming gas to the first discharge space from a mixed gas supply means to form a high-frequency electric field between the pair of roll electrodes. The second discharge space is located at the outer periphery of one of the roll electrodes and used in a post-treatment step of performing post-treatment by introducing the base material on which the functional film is formed thereinto and introducing a mixed gas (2) containing the discharge gas and a post-treatment gas between a counter electrode and the roll electrode to form a high-frequency electric field between the counter electrode and the roll electrode.