A semiconductor device includes a monitoring circuit receiving a first supply signal and generating a flag signal by monitoring a second supply signal provided to a monitored circuit; a watchdog circuit disposed in the monitoring circuit, receiving the first supply signal, and generating a watchdog signal; and a comparison circuit receiving the flag signal and the watchdog signal and generating a first signal which has a first level under an abnormal condition and a second level under a normal condition. The second level is different than the first level. The watchdog circuit sets the watchdog signal to a third level when the level of the first supply signal is lower than a first reference level. When the watchdog signal has the third level, the comparison circuit allows the first signal to have the first level regardless of the level of the flag signal.