There is provided a semiconductor device that can follow a fast voltage change such as a large voltage drop occurring at the time of rapid load fluctuation. The semiconductor device includes a voltage sensor which monitors a power supply voltage at a sampling speed higher than the assumed frequency of power supply voltage fluctuation and outputs a voltage code value, a voltage drop determination circuit which determines, from the voltage code value, that a voltage drop causing a malfunction of a system occurs, and outputs a clock stop signal, and a clock control circuit which controls clock stop, restart, and frequency change.