The method of manufacturing a semiconductor device include: forming conductive patterns in interlayer spaces between interlayer insulating layers, the conductive patterns being separated from each other by a slit passing through the interlayer insulating layers, wherein the conductive patterns include a first by-product; generating a second by-product of a gas phase by reacting the first by-product remaining in the conductive patterns with source gas; and performing an out-gassing process to remove the second by-product.