A semiconductor device includes a voltage sensor which samples a power supply voltage at a speed faster than fluctuations in the power supply voltage and encodes the power supply voltage into a voltage code value. A voltage drop determination circuit detects a voltage drop based on the voltage code value, and a clock control circuit generates a clock. The clock control circuit stops the clock when the voltage drop determination circuit detects the voltage drop. The voltage drop determination circuit includes a prediction computation circuit which looks ahead a voltage value from a history of the voltage code value and predicts a variation value, and the prediction computation circuit includes a circuit for masking a prediction value if a differential value of the prediction value is continuously negative for a predetermined cycle.