Even when a positive/negative surge is applied to a power-supply line, occurrence of malfunction in a semiconductor device is lessened. In an in-vehicle semiconductor device, regulators convert an externally-supplied voltage to generate voltages. A voltage monitoring unit includes a voltage monitoring circuit and a switch, and monitors first to third voltages. An internal circuit has a processor operated by the voltage, and an oscillator operated by the voltage and generating a clock signal to be provided the processor. The voltage monitoring unit stops providing the clock signal to the processor when the voltage level of at least any one of the first to third voltages is below a set value. The voltage monitoring unit provides the clock signal to the processor when the voltage levels of all the first to third voltages exceed the set value.