When overdriving a first power supply voltage supplied to a sense amplifier, a line for the first power supply voltage and a line for a second power supply voltage which is higher than the first power supply voltage are connected to each other by a first transistor, thereby boosting the first power supply voltage. When the first power supply voltage drops upon activation of the sense amplifier, the line for the first power supply voltage and the line for the second power supply voltage are connected to each other by a second transistor, thereby increasing the current supply capability. The first transistor and the second transistor are fully driven to operate as switches.