A CMOS circuit is a necessary component for a semiconductor integrated circuit because a CMOS circuit has low power consumption and can operate at high speed and can be highly integrated.  On the other hand, in recent years, in accordance with miniaturization of a MOS transistor, an increase of power consumption at the time when an increase of power consumption in a non operating state (power consumption in a standby period, hereinafter also referred to as standby power) due to an increase of leakage current (also referred to as off state current, subthreshold current or the like) has been a problem.  For example, in a silicon MOS transistor whose channel length is miniaturized to approximately 0.1 μm or less, the value of drain current cannot be made zero when a potential between a gate and a source is set to threshold voltage or less. 
To prevent an increase of the standby power due to the leakage current, a technique using a switching transistor has been proposed (for example, see Patent Document 1).  The technique disclosed in Patent Document 1 is as follows: a switching transistor having small leakage current compared to a CMOS circuit is provided between a power supply and the CMOS circuit; the switching transistor is turned off when the CMOS circuit is not in operation so that standby power is decreased.