A memory device comprises a memory cell array including a first memory cell disposed on a substrate and a second memory cell above the first memory cell; a first word line connected to the first memory cell and a second word line connected to the second memory cell, the second word line disposed above the first word line; and a word line defect detection circuit configured to monitor a number of pulses of a pumping clock signal while applying a first voltage to the first word line to detect a defect of the first word line. The voltage generator is configured to apply a second voltage different from the first voltage to the second word line for programming the second memory cell when the number of pulses of the pumping clock signal is smaller than a reference value.