There are provided a semiconductor device and a manufacturing method thereof. The semiconductor device includes: a stack structure including a plurality of interlayer insulating layers and a plurality of gate conductive layers, which are stacked in an alternating manner; at least one support structure penetrating the stack structure in a substantially vertical manner, the at least one support structure being formed in a contact region; and a contact plug penetrating the stack structure in a substantially vertical manner, the contact plug being formed in the contact region, the contact plug being connected to a contact pad that is disposed on the bottom of the stack structure. The at least one support structure is formed of an oxide layer.