A semiconductor device includes: a first stack structure; a second stack structure; a slit insulating layer located between the first stack structure and the second stack structure, the slit insulating layer extending in a first direction; a conductive plug located between the first stack structure and the second stack structure, the conductive plug including a first protrusion part protruding to the inside of the slit insulating layer; and an insulating spacer surrounding a sidewall of the conductive plug.