A semiconductor memory device includes a charge pumping circuit, a level sensor, an oscillator, and a pumping control signal generator. The charge pumping circuit performs a negative-pumping operation to an external power in order to generate an internal voltage having a level lower than the external power. The level sensor senses a level of the internal voltage corresponding to a level of an adjusted reference voltage during a refresh mode. The oscillator generates a period signal in response to a sensing signal of the level sensor. The pumping control signal generator controls the operation of the charge pumping circuit in response to the period signal.