A semiconductor memory device includes a peripheral circuit structure including a peripheral circuit insulating layer, a middle connection structure on the peripheral circuit insulating layer, the middle connection structure including a middle connection insulating layer, and a bottom surface of the middle connection insulating layer is in contact with a top surface of the peripheral circuit insulating layer, stack structures on sides of the middle connection structure, and channel structures extending vertically through each of the stack structures, wherein at least one side surface of the middle connection insulating layer is an inclined surface, a lateral sectional area of the middle connection insulating layer decreasing in an upward direction oriented away from the peripheral circuit insulating layer.