An undercoat agent which is used for phase separating a layer including a block copolymer having a plurality of blocks bonded formed on a substrate, wherein the undercoat agent contains a resin component, the resin component includes a structural unit having an aromatic ring and a structural unit having no aromatic ring, and the resin component includes a group which can interact with the substrate and does not include a 3 to 7-membered, ether-containing cyclic group; and a method of forming a pattern of a layer containing a block copolymer, including: a step (1) in which the undercoat agent is applied to a substrate to form a layer containing the undercoat agent; a step (2) in which a layer containing a block copolymer having a plurality of blocks bonded is formed on a surface of the layer containing the undercoat agent, followed by a phase separation of the layer containing the block copolymer; and a step (3) in which a phase containing at least one block of the plurality of blocks constituting the block copolymer is selectively removed.