A power-up signal generating circuit that prevents repeatedly generating a power-up signal even when there is noise on an external voltage. The power-up signal generating circuit includes a level detector, a level comparator, and a reentry protector. The level detector is configured to deactivate a first level detection signal when a level of an external voltage increases above a upper limit reference voltage. The level comparator is configured to deactivate a second level detection signal when the level of the external voltage increases above a lower limit reference voltage. The reentry protector is configured to activate the power-up signal in response to the second level detection signal and deactivate the power-up signal in response to a deactivation of the first level detection signal.