Primer and pattern forming method for layer including block copolymer

A method of forming a pattern of a layer containing a block copolymer, the method comprising the steps of:n(1) coating an undercoat agent on a substrate, thereby forming a layer composed of the undercoat agent,(2) forming a layer containing a block copolymer having a plurality of polymers bonded on the surface of the layer composed of the undercoat agent, and subjecting the layer containing the block copolymer to phase separation, and(3) selectively removing a phase of at least one polymer of the plurality of copolymers constituting the block copolymer from the layer containing the block copolymer, whereinthe undercoat agent comprises a resin component,the resin component comprises a structural unit derived from an aromatic ring-containing monomer and a structural unit derived from an aromatic ring non-containing monomer which is a (meth)acrylic compound or a vinyl compound containing at least 1 atom selected from the group consisting of N, O, Si, P and S, provided that 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, andthe undercoat agent comprises a polymerizable monomer and at least one photosensitive polymerization initiator selected from the group consisting of a cationic polymerization initiator and a radical polymerization initiator.