To provide a memory device which can maintain data accurately even when memory characteristics of a memory element deteriorate over time. The memory device includes a memory cell 100, a reading circuit 103, a power supply line 104, a first signal line 105, a second signal line 102, and an output terminal 106. The memory cell 100 includes a memory element 108, the resistance value of which is changed and holds data by utilizing the resistance value of the memory element 108. The reading circuit 103 reads data held in the memory cell 100. The output terminal 106 outputs a potential of the power supply line 104 or a potential corresponding to the data held in the memory cell 100 in accordance with the resistance value of the memory element 108. The reading circuit 103 includes a transistor 109 having first to fourth terminals. The threshold voltage of the transistor 109 is controlled by supplying a potential to a channel region through the fourth terminal.