A power supply voltage control apparatus capable of freely setting a clock period setting margin according to a system clock frequency, and capable of converging power supply voltage to minimum power supply voltage where normal operation is possible in a short period of time without errors in operation of internal circuits in response to changes in the system clock frequency is provided. Power supply voltage control apparatus is provided with a first frequency-divider that frequency-divides the system clock at a first frequency-diving ratio, a second frequency-divider that frequency-divides an output of a voltage control oscillator at a second frequency-dividing ratio, a phase comparator/frequency comparator that carries out a phase comparison/frequency comparison on the respective output signals of the first and second frequency-dividers, and a controller. Frequency-dividing ratios of the first and second frequency-divider are set by a second controller according to an operation mode signal corresponding to the system clock frequency.