An undercoat agent including a block copolymer having a plurality of blocks bonded formed on a substrate. The undercoat agent contains a resin component that 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. The method includes applying an undercoat agent to a substrate to form a layer containing the undercoat agent; forming a layer containing a block copolymer having multiple blocks bonded on a surface of the layer containing the undercoat agent, followed by a phase separation of the layer containing the block copolymer; and selectively removing a phase containing at least one block of multiple blocks constituting the block copolymer.