A substrate bias voltage detection unit compares a level of a substrate bias voltage with a reference voltage in response to a self-refresh signal, an idle signal, and a refresh count signal so as to output an oscillating driving signal, enables the oscillating driving signal when the substrate bias voltage is equal to or higher than a first level in a normal mode, disables the oscillating driving signal when the substrate bias voltage is at a second level in a self-refresh mode, and disables the oscillating driving signal when the substrate bias voltage is at a third level in the self-refresh mode. An oscillation unit outputs an oscillating signal according to the oscillating driving signal. A voltage pumping unit controls pumping of the substrate bias voltage according to an output signal of the oscillation unit and then outputs a pumped substrate bias voltage.