A memory device includes memory blocks, each including memory cells, and peripheral circuits that control the memory blocks and execute an erase operation for each of the memory blocks. Each memory block includes word lines stacked on a substrate, channel structures extending perpendicular to an upper surface of the substrate and penetrating through the word lines, and a source region disposed on the substrate and connected to the channel structures. During an erase operation in which an erase voltage is input to the source region of a target memory block, the peripheral circuits reduce a voltage of a first word line from a first bias voltage to a second bias voltage at a first time and reduce a voltage of a second word line, different from the first word line, from a third bias voltage to a fourth bias voltage at a second time different from the first in time.