A thin film formation method is used for forming a thin film by providing a conductance valve on an exhaust path connecting a depressurizable processing chamber and a vacuum pump, arranging a processing object substrate inside the processing chamber, performing once or plural times a cycle including a first step of supplying a first reactive gas and a second step of supplying a second reactive gas into the processing chamber during a film formation processing period to cause a chemical reaction between the first reactive gas and the second reactive gas, and using the chemical reaction to form the thin film on the substrate. The thin film formation method includes a first process of supplying into the processing chamber a predetermined gas by a specified flow while exhausting the inside of the processing chamber, and determining a reference value that is equal to a valve opening level of the conductance valve causing pressure inside the processing chamber to substantially match a specified value, the first process being performed during a preparation period before the film formation processing period starts; and a second process of maintaining the valve opening level of the conductance valve at the reference value at least during the first step and the second step of the cycle performed during the film formation processing period.