An aging monitoring circuit of a semiconductor memory device includes a threshold voltage sensing part including an aging monitoring transistor, enabled in response to activation of an aging monitoring signal, and generating a sensing threshold signal, a level of the sensing threshold signal depending on a threshold voltage of the aging monitoring transistor, a reference threshold storage part receiving the sensing threshold signal generated in response to activation of a reference sensing signal and storing a reference threshold voltage, a level of the reference threshold voltage depending on the level of the sensing threshold signal, and a level comparing part enabled in response to the activation of the aging monitoring signal and generating an aging flag signal, a logic state of the aging flag signal depending on a comparison result between the level of the sensing threshold signal and the level of the reference threshold voltage.