A semiconductor device includes: a power switch connecting an internal power supply in which power is not shut down and an internal power supply in which power is shut down; and an internal voltage determining circuit for determining voltage of the internal power supply in which power is shut down. Voltage of the internal power supply in which power is shut down is generated from external power supply voltage by using a regulator circuit. When the power of the internal power supply is interrupted, the power switch is turned off, the regulator circuit is turned off, and an output of the regulator circuit is shorted to a ground potential. When power of the internal power supply is resumed, the regulator circuit is turned on, shorting is cancelled, increased voltage of the internal power supply is determined, operation of a circuit block is started, and the switch is turned on.
