A NAND type flash memory device includes a semiconductor substrate, word lines, first and second selection lines, tunnel insulation layers, and selection gate insulation layers. The semiconductor substrate includes a memory transistor region and a selection transistor region. The word lines are arranged in the memory transistor region of the semiconductor substrate, and the selection lines are arranged in the selection transistor region of the semiconductor substrate. The tunnel insulation layers are interposed between the word lines and the semiconductor substrate, and the selection gate insulation layers are interposed between the selection lines and the semiconductor substrate and have a thinner thickness than the thickness of the tunnel insulation layers. Also, the selection gate insulation layers have a thinner thickness in their center region than in their edge portions.