A semiconductor device includes a stacked structure including a first region in which conductive layers and the insulating layers are stacked alternately with each other, and a second region in which sacrificial layers and the insulating layers are stacked alternately with each other, a first slit structure located at a boundary between the first region and the second region and including a first through portion passing through the stacked structure and first protrusions extending from a sidewall of the first through portion, a second slit structure located at the boundary and including a second through portion passing through the stacked structure and second protrusions extending from a sidewall of the second through portion and coupled to the first protrusions, a circuit located under the stacked structure, and a contact plug passing through the second region of the stacked structure and electrically connected to the circuit.