A thin film formation method to form a silicon film containing an impurity on a surface of an object to be processed in a process chamber that allows vacuum exhaust includes alternately and repeatedly performing a first gas supply process in which a silane-based gas composed of silicon and hydrogen is supplied into the process chamber in a state that the silane-based gas is adsorbed onto the surface of the object to be processed and a second gas supply process in which an impurity-containing gas is supplied into the process chamber, to form an amorphous silicon film containing an impurity. Accordingly, an amorphous silicon film containing an impurity having good filling characteristics can be formed even at a relatively low temperature.