An undercoat agent usable in phase separation of a layer formed on a substrate, the layer containing a block copolymer having a plurality of polymers bonded, the undercoat agent including a resin component, and 20 mol % to 80 mol % of all the structural units of the resin component being a structural unit derived from an aromatic ring-containing monomer; and a method of forming a pattern of a layer containing a block copolymer, the method including: step (1) coating the undercoat agent on a substrate (1), thereby forming a layer (2) composed of the undercoat agent, step (2) forming a layer (3) containing a block copolymer having a plurality of polymers bonded on the surface of the layer (2) composed of the undercoat agent, and subjecting the layer (3) containing the block copolymer to phase separation, and step (3) selectively removing a phase (3a) of at least one polymer of the plurality of copolymers constituting the block copolymer from the layer (3) containing the block copolymer.