A method of manufacturing a fuel tank (20) includes: an introducing process in which a molten resin sheet (34) formed with a tank body (22) is introduced in a forming mold (42); a shaping process in which a movable member (44) abuts against the molten resin sheet (34) to form a molten resin projection (36) projected in a thickness direction of the molten resin sheet (34); an arranging process in which the molten resin projection (36) is inserted in an attachment hole (30) provided in an attachment part (26), and the attachment part (26) is arranged on the molten resin sheet (34); and a machining process in which the molten resin projection (36) is pressurized by a pressurizing member (46), so as to bulge out a portion of the molten resin projection 36 to an outer peripheral side.