There are provided a semiconductor device and a manufacturing method thereof. The semiconductor device includes: a stack structure including a plurality of conductive patterns and a plurality of interlayer insulating layers, which are alternately stacked on a substrate; a plurality of channel structures extending in a first direction substantially perpendicular to the substrate to penetrate the stack structure; at least one first slit extending in a second direction substantially horizontal to the substrate while penetrating conductive patterns for select lines among the plurality of conductive patterns; a second slit extending in the second direction while penetrating the conductive patterns for select lines; and a plurality of support structures disposed on a bottom of the second slit, the plurality of support structures penetrating conductive patterns for word lines among the plurality of conductive patterns.