A film formation method of forming a silicon oxide film on a surface of an object to be processed in a process chamber includes absorbing a seed gas comprising a silane-based gas on the surface of the object to be processed by supplying the seed gas into the process chamber, forming a silicon film having an impurity by supplying a silicon-containing gas as a material gas, and an addition gas including the impurity into the process chamber, and oxidizing the silicon film to convert the silicon film into the silicon oxide film. Accordingly, the silicon oxide film having the high density and the high stress is formed on the surface of the object to be processed.