A self-assembled pattern forming method in an embodiment includes: forming a guide pattern on a substrate; forming a layer of a first polymer; filling a first block copolymer; and phase-separating the first block copolymer. The guide pattern includes a first recessed part having a depth T and a diameter D smaller than the depth T, and a second recessed part having a width larger than double of the diameter D. The first block copolymer has the first polymer and a second polymer which are substantially the same in volume fraction. By phase-separating the first block copolymer, a cylinder structure and a lamellar structure are obtained.