A semiconductor device includes a first gate stack structure and a second gate stack structure, which face each other; channel patterns extending in a first direction to penetrate the first gate stack structure and the second gate stack structure; memory patterns extending along outer walls of the channel patterns; and a source contact structure disposed between the first gate stack structure and the second gate stack structure, wherein the source contact structure includes a vertical part extending in the first direction and horizontal protrusion parts protruding toward a sidewall of the first gate stack structure and a sidewall of the second gate stack structure from both sides of the vertical part.