A protective film forming method is provided. In the method, substantially an entire surface of a silicon-containing underfilm is terminated with fluorine by supplying a fluorine-containing gas to the silicon-containing underfilm formed on a substrate having a surface including a plurality of recesses and a flat surface provided between the adjacent recesses. A surface of the silicon-containing underfilm formed on the flat surface of the substrate is nitrided by supplying a nitriding gas converted to plasma to the silicon-containing underfilm terminated with fluorine such that a silicon adsorption site is formed on the surface of the silicon-containing underfilm formed on the flat surface of the substrate. A silicon-containing gas is adsorbed on the silicon adsorption site by supplying the silicon-containing gas to the silicon-containing underfilm.