A semiconductor memory device can generate an under_drive voltage that maintains a predetermined level stably even in case of a change in the operation mode of the semiconductor memory device or the level of an external power supply voltage. The semiconductor memory device, which includes an external power supply voltage detector configured to detect a level of an external power supply voltage to generate the external voltage detection signal, an under_drive voltage detector configured to detect a voltage level of an under_drive voltage to generate the under_drive voltage detection signal, and an under_drive voltage generator configured to generate the under_drive voltage in response to the under_drive voltage detection signal with a variable driving force in response to the external voltage detection signal.