An apparatus for processing a substrate for processing a substrate includes: one or more processing chambers, a plasma generator, and a controller. The controller causes (a) providing a substrate having an etching region and a patterned region on the etching region; (b) forming an organic film on a surface of the substrate; and (c) etching the etching region through the patterned region using plasma generated from a processing gas. The step (b) includes (b1) supplying a first gas containing an organic compound to the substrate to form a precursor layer on the substrate, and (b2) supplying a second gas containing a modifying gas to the substrate and supplying energy to the precursor layer and/or the second gas to modify the precursor layer. In (b2), the precursor layer is modified by plasma generated from the second gas.