The present invention provides a method for forming a resist under layer film used in a lithography process, comprising: a process for applying a composition for forming a resist under layer film containing an organic compound having an aromatic unit on a substrate; and a process for heat-treating the resist under layer film applied in an atmosphere whose oxygen concentration is 10% or more at 150° C. to 600° C. for 10 to 600 seconds after heat-treating the same in an atmosphere whose oxygen concentration is less than 10% at 50 to 350° C. There can be provided a method for forming a resist under layer film having excellent filling/flattening properties so that unevenness on a substrate can be flattened even in complex processes such as multi-layer resist method and double patterning.