To provide a semiconductor device having a voltage monitoring circuit which can reduce power consumption by changing a power source voltage outputted from a step-down circuit in response to an operation mode.

The semiconductor device having the voltage monitoring circuit comprises the step-down circuit 20 for converting the power source voltage to an output voltage in response to a control signal to supply the output voltage to an internal module of the device via a power supply line, an AD converter 30 for converting the output voltage from the power source line to output a signal indicating the output voltage value, and a voltage monitoring circuit 10 having a control register for holding the value corresponding to the selected operation mode to generate a control signal based on the compared result of the output signal of the AD converter with a value held in the register to supply the control signal to the circuit 20 for regulating the output voltage to an optimum output voltage.