A resin composition for forming a phase-separated structure contains a block copolymer that is formed of a first block, a second block, and a third block, which are bonded to one another. The structure of a constituent unit of a polymer constituting the first block is identical to the structure of a constituent unit of a polymer constituting the third block. The number-average molecular weight (Mn3) of the polymer constituting the third block is smaller than the number-average molecular weight (Mn1) of the polymer constituting the first block.