A thin film forming method which forms a seed film and an impurity-containing silicon film on a surface of an object to be processed in a processing container configured to be vacuum exhaustible, the thin film forming method includes: performing a first step which forms the seed film formed of a compound of silicon, carbon and nitrogen on the surface of the object by supplying a seed film raw material gas comprising an aminosilane-based gas into the processing container; and performing a second step which forms the impurity-containing silicon film in an amorphous state on the seed film by supplying a silane-based gas and an impurity-containing gas into the processing container.