A pad structure including a plurality of staircase structures is provided. The staircase structures are disposed on the substrate. Each of the staircase structures includes a plurality of conductor layers and a plurality of dielectric layers that are alternately stacked. Two adjacent staircase structures are connected with each other by sharing the conductor layers and the dielectric layers and are arranged in parallel along a first direction. One of the two adjacent staircase structures includes at least one staircase portion that gradually decreases in height along a second direction, and the other of the two adjacent staircase structures includes at least one staircase portion that gradually decreases in height along a direction opposite to the second direction.