A method of forming a thin film containing a doping element in a vacuum atmosphere, which includes: supplying a source gas into a processing vessel being under the vacuum atmosphere through a source gas supply unit such that a source of the source gas is adsorbed onto a substrate in the processing vessel; repeating, a plurality of times, a sequence of operations of supplying a doping gas containing the doping element into the processing vessel through a doping gas supply unit, followed by sealing the doping gas inside the processing vessel, followed by vacuum-exhausting the processing vessel; supplying a reaction gas into the processing vessel through a reaction gas supply unit such that the reaction gas reacts with the source to produce a reaction product; and replacing an internal atmosphere of the processing vessel, the replacing being performed between the operations.