A method of producing a structure containing a phase-separated structure, including using a resin composition for forming a phase-separated structure including a block copolymer having a period L0 and an ion liquid containing a compound (IL) having a cation moiety and an anion moiety to form a BCP layer containing a block copolymer and having a thickness of d (nm) on a substrate; and vaporizing at least a part of the compound (IL), and phase-separating the BCP layer to obtain a structure containing a phase-separated structure, wherein, in step (i), the BCP layer is formed such that the period L0 (nm) of the block copolymer and the thickness d (nm) of the BCP layer satisfies the following formula (1): Thickness d/Period L0=n+a   (1) wherein n represents an integer of 0 or more; and a is a number which satisfies 0