There is provided a technique that includes filling a concave portion formed on a surface of a substrate with a first film and a second film by performing: (a) forming the first film having a hollow portion using a first precursor so as to fill the concave portion formed on the surface of the substrate; (b) etching a portion of the first film which makes contact with the hollow portion, using an etching agent; and (c) forming the second film on the first film of which the portion is etched, using a second precursor, wherein (b) includes performing, a predetermined number of times: (b-1) modifying a portion of the first film using a modifying agent; and (b-2) selectively etching the modified portion of the first film using the etching agent.