When an object to be processed is transferred into a process chamber capable of keeping a vacuum and an interior of the process chamber is kept in a vacuum state, the film formation method includes performing forming a first ZrO film on the object to be processed by supplying a zirconium material and an oxidizing agent in the order listed above into the process chamber and forming a second ZrO film doped with Si on the object to be processed by supplying the zirconium material, a silicon material, and the oxidizing agent in the order listed above into the process chamber, in such a way that a number of times the forming the first ZrO film is performed and a number of times the forming the second ZrO film is performed are adjusted, respectively, to form a zirconia-based film having a predetermined film thickness while controlling a Si concentration in the film.