A memory device includes a memory cell array including a plurality of word lines, a first string select line above the plurality of word lines, and a second string select line between the first string select line and the plurality of word lines, and a controller. During an operation of reading data of a first memory cell connected to a first word line among the plurality of word lines, the controller is to supply a first voltage to the first string select line and to supply a second voltage to the second string select line, the second voltage being greater than the first voltage.